PS can't open 8k mp4 created by Canon R5. You get error "Could not open "filename.mp4" because the video file could not be opened. The wierd thing is that LR and Windows OS can open the file with no issue. Also while in LR, I can't "edit in" anything. All normal target apps are grayed out. Support was able to recreate the issue with one of my files.
Have you checked to see if Camera RAW has support for the Canon R5 8k RAW files yet? Also, since you said that Lightroom can see it and not Photoshop, have you updated Photoshop to the latest version yet?