Camera Raw

If you want to use ACR directly, you must do so from within Photoshop (or double click on a raw). Lightroom Classic has always used ACR to render images when using the Edit in Photoshop command. If the two are not on version parity, you will be told this and asked which to use, otherwise, the Lightroom Classic command will send the raw and processing instructions to ACR, it will render them and all you see in Photoshop is the end results of that processing.

As explained by @Rob_Cullen , the Camera Raw interface will not appear when you use Edit in Photoshop.

If the image is a raw file, it will open silently in Camera Raw (which is a plugin for Photoshop and Bridge), where any adjustments from Lightroom will be applied. A new file will then be rendered from the raw file, and open in Photoshop.

Correct me if I'm wrong, but I get the impression that you routinely send photos to Photoshop from Lightroom directly after importing. That is not a good workflow – you should do as much editing as possible in Lightroom, and only use Edit in Photoshop to do things that are not possible in Lightroom.

Editing in Lightroom is non-destructive, the edits are saved as text, and only applied to a new file when you export, or use Edit in Photoshop. The original remains unchanged.

Any edits you do in Photoshop are baked into the file itself, and cannot be undone. (unless you use Smart objects)

Also note that Camera Raw is an integral part of Lightroom. You can do the exact same edits in Lightroom as in Camera Raw, so even if you could, there would be no point in sending an image from Lightroom to Camera Raw.

Which version of Lightroom?  Classic or Cloud?

Which Version Number of Lr?

What file-type is the photo you are editing?  RAW (NEF,CR2,ARW, etc) or RGB (JPG,TIF,etc)?

"CAMERA RAW doesn't automatically pop up as a window as it used to." It never did from Lr-CLassic!

The Camera Raw Interface will only open if you 'Open' the file from Bridge or a File Browser.

Or- ACR can be used if you 'Open as Smart Object' from Lr-Classic to Ps and [double-click] the Layer in Ps.