Date/time changes to today's date after Develop Module or metadata edit

What you are seeing is not new, and it’s probably working exactly as designed. The real issue is that you want to look at and use the appropriate date metadata fields. What’s not obvious is that there are actually several date fields and they record different kinds of dates. And Metadata Date is not at all what you are interested in. The short version is: If you want to preserve the date that the original photo was taken, you want to add Date Time Original metadata.

Documentation on Metadata Date seems to be difficult to find, because it isn’t exposed to most applications. Metadata Date seems to be about — and only about — the last time XMP metadata was changed outside of the Lightroom Classic catalog. It is not related to the capture or file creation dates in any way. The one explanation I found about Metadata Date is from this post on the Lightroom Queen forums, which says:

Metadata Date will only appear as a visible field if metadata is saved to file and it records the last date this was done.

The date fields for photos confuse many people, it’s not just you. The post linked above describes what the other dates are for. People are usually most interested in Date Time Original, which is what a digital camera would embed into the photo at the moment it’s captured.

The pictures below are some examples of how to think through dates for scanned images, as those dates are shown in Lightroom Classic. I set the Metadata panel view to Default; if your Default view looks different, that’s probably because I customized how fields are displayed in that view. (After taking these screen shots, I decided to customize it again to move the Metadata Date down in the list, to group it with the other dates.)

For the digital camera image (left):

Date Time is like the Date Modified date in the computer file system (in our case macOS Finder), when the file was last changed. Not directly related to when the photo was taken, it’s only about the file containing the image.
Capture Date/Time is like the Date Created date in a computer file system; for example, when you choose File > New in a word processor and choose File > Save As, creating a file that never existed before. Again — this date is about the file, not the image in it. For digital camera images this is typically set to be the same as Date Time Original.
Date Time Original is when the photo was taken, and is not changed by normal edits. Because this is about the image, not the file containing it.
Date Time Digitized is when the photo was rendered; for digital cameras of course this is the same moment it was captured. For scanning software that supports this field, it is the time of scan.

For the scanned image (right):
Date Time Original and Date Time Digitized are empty because many common scanning applications don’t record them, because they obviously can’t automatically know when the original film frame was shot. That’s why a film frame I shot in 1985 gets file dates from 2003, when I scanned it. A flatbed scanner can’t even assume it’s scanning a photo.

Next, the difference betwen (most) scanner metadata, and scanner metadata enhanced for photos:

In the picture above, the left image is from an image scanned with typical scanning software. It records a scan with OS file system information, but no photographic metadata.

The right image is how I scan film. My scanning software (VueScan) has a Date field, and if I fill it in before saving a scan, VueScan embeds the date I entered into Date Time Original, so that date appears in applications such as Lightroom Classic as if that photo was taken with a digital camera, and it can now be sorted by date with digital camera images. After capture, Date Time Original should never change, which is important because other date metadata fields can change, especially the fields recording modification dates.

Notice the differences between the dates on the right and what they mean:

Capture Date/Time. When the scan was written to a new image file in the computer.

Date Time Original. When the photo was taken in the film camera, entered by me.

Date Time Digitized. When the film was scanned.

If I create a new file from that image file (e.g. if it’s open in Photoshop and I choose File > Save As), Capture Date/Time changes because that is an OS file creation date, but the other two dates don’t change, because you wouldn’t want them to.

Of course, many scanning apps don’t let you add a date that writes to Date Time Original. Or, maybe I forgot to fill in the date in VueScan when scanning, or left the wrong date in the field when scanning images from a later date. So I need to change Date Time Original later in the original scan file. One way to do that is in Lightroom Classic choose Metadata > Edit Capture Time, but if you want that date to change in the original file you next choose Metadata > Save Metadata to File. You can also use the plug-in Capture Time to Exif, which you can read about in this article:
How do I change the photo dates on scanned / photographed images?

Metadata Date changes only when the metadata is actually written back out to the original file (for non-camera raw images) or XMP metadata sidecar file (for camera raw images). Simply making an edit doesn’t change the field, unless you enabled the Automatically Write Changes into XMP setting.

Notice that there are five date/time fields with three different values, which are all true. If this file was opened in an image editor such as Photoshop and saved as a new file with File > Save As, then Capture Date/Time (which is Date Created in macOS) would change, so that one image file might then store four different valid date/times in five metadata fields.

So, again: If you want to preserve the date that the original photo was taken, Metadata Date is absolutely not the field you should be looking at. You want to add Date Time Original metadata to the scan.