I edited an HEIC file, then exported into the same folder with the same filename with a .JPG extension. What I'm left with in the Grid view is a single thumbnail that indicates a file extension of "HEIC+JPG", rather than two files. In order to delete the original HEIC file, I have to to the Finder or some other program that understands files.
I'm guessing that the Grid was thinking it useful to display these two files as one. (It's not.) But how do I tell Lightroom to display these as two separate files?
LR is weird, treating JPEGs as sidecars of a number of non-raw file types -- TIFF, HEIC, MP4, perhaps others. So check the option Preferences > General > Treat JPEG Files Next To Raw Files As Separate Photos. Then in the Folders panel, right-click the affected folders and do Synchronize Folder to cause the JPEGs to be imported as separate photos.