Do have any links to LR forum threads in which this issue became apparent in LR 7.4?
No, I couldn't find any information about this issue. I just meant that I have had this issue for a while (including in previous versions), so it's not specific to the 7.5 release and updates haven't corrected it. I can't remember when it started.
Can you show a screenshot of the Lightroom folder panel (with the folder that is doing this selected) and a screenshot of the consequent import dialog? It sounds like that for some reason the source folder gets deselected in the import dialog, and instead the root of your computer is selected.
It seems that I've solved the problem. I'm not sure what the cause was but this is how I fixed it:
I individually synchronized all the subfolders of the folder that was causing the issue and eventually narrowed the problem down to 1 subfolder. I removed that folder from the catalogue (right click "Remove...") then synchronized the parent folder to add it back in and it's now working as expected.