Just upgraded to 11.2, and Sychronising a folder on an External f: Hdd drive (a single folder 1 level down) containing 3 video files). The scan correctly identifies that there are 3 files to synchronise. I choose not to show the import folder.
After clicking synchronise, the progress bar at top left starts to fill in. Shaortly after that, a second task starts, and my D drive (also external ssd) starts to synchronise. There are and should be no files on the D Drive in this catalog.
This is repeatable, (even after a restart of LR Classic and a re-boot)
If I chose the Import button and select the same folder, then the three files import perfectly as normal.
I have seen more often that Synchronize Folder should always be combined with showing the import dialog, or else things may not go as expected. I am not sure if this has ever been identified and logged as a bug, but my advice is to always show the import dialog.
Because of the rather odd way I work with my daily supply of new genealogy image files, I've been using synchronise folder (without the import dialog) probably 8 to 10 times a day for several years, and never once had a problem of any type, never mind like this, Lr has been very reliable in this respect (for me).
I know of course that that does not mean others will not have seen issues, maybe I've just been lucky. Usually it is only loooking at 4 to 6 files needing syncing, so that may not be stressing the sync.