For reasons that I do not understand, LRC tends to shift from whatever sort order you had to date or something like that.
To "fix" this (at least short term), Go to Library mode, then Grid mode and then you can set the sort order to "Name." That should place the PSed image next to the original image where you can Stack them or even just FIND them! ;>)
Do you have the 'Previous Import' collection selected? That collection defaults to 'Added Order' sort, so that explains why a newly added file (from Photoshop) gets added to the end of the grid. It should not happen in any other collection or folder. The chosen sort order should stick (and does in Lightroom Classic 10.3 on my Mac).