Currently running the latest Lightroom Classic on a new MacBook Air with up to date MacOS.
Handling files the same way I have done a million times before:
1. Import raws into Photo Mechanic and created XMP metadata files alongside them.
2. Added images to Lightroom Classic catalog with no metadata or develop preset options chosen.
3. Images show no metadata and in Develop, the history shows Reset Settings as a step that has been taken immediately after import.
If I ask LR to read metadata from files, the history pane shows Reset Settings as being applied again.
Well, since it's been a couple of days since you posted, I'll take a stab.
I understand you are trying to get the contents of XMP side-car files created in Photo Mechanic read into LrC when you import their associate RAW files.
As far as I know, Adobe does not publish the specifications of how to create XMP side car files for RAW images. Consider it proprietary. I have successfully loaded RAW files with XMP into LrC from LR 6.
In other words, you may have been lucky in the past with LrC reading Photo Mechanic but either Photo Mechanic changed someting, and/or LrC did, so LrC cannot read them. Unfornately, this means reviewing successful and unsuccesful XMP files to see what has changed. You will then have to investigate modifying the new XMP files so they will load.
As an alternative, you can look at using a utility like EXIFTOOL to add the desired metadata directly to the RAW files before importing into LrC. This is what I do but you have to comfortable using command line to do this.
I vaguely remember a long thread about this problem. If I remember correctly, the images were on a network disk, but I'm not sure that this was a factor. I believe the answer was that Lightroom is case-sensitive and writes the .XMP extension in lower case (so as ".xmp"), while Photo Mechanic writes it in UPPERCASE (".XMP"). That may explain why Lightroom ignores the Photo Mechanic sidecar files.
To build on Johan's reply, LR will get confused if the photos and sidecars are stored on a case-sensitive file system. Are your photos stored on the Macbook's internal drive, external drive, or a network drive?
Also, there have been a number of reports of .xmp case-confusion on Synology network drives. Some users have resolved that by changing the drive from the deprecated AFP protocol to SMB. Others have resolved it by changing permissions. See these threads:
I'm having a very similar problem on a 2016 Macbook Pro with the latest version of both Big Sur and Lightroom classic.
I worked on a bunch of ARW files shot on a Sony A7iii and saved the metadata. I needed to rename the files and move them to another folder. I used finder to batch rename the files, so the XMP still matches the ARW. When I re-import I can't see any of the edits I made. Even after 'read metadata' the photos remain unchanged. Opening the XMP file with a text editor, I can see my work is still there but lightroom won't read it.
Based on Johnrellis' post, I was using a case sensitive external drive. But after moving to my internal hard drive the problem persists.
Upload one of the problem photos and its .xmp sidecar to Dropbox or similar and post the sharing link here. We can see if the .xmp is read by other LR configurations.
Ugh, nevermind. This is user error because I'm new to all this. I was expecting to see the history of the edits in the left column. But when I look to the actual settings on the right, all my changes are saved there. I just can't see the history. Sorry about that.
Unfoturnately, as you've discovered, LR doesn't record the history in .xmp metadata.