I have many many thousands, and I've had to recategorize three times due to broken syncs. In the greater scheme of the shortcomings of FTM's media management, the goofy shift-clicking is just a minor nuisance, but I'm glad that Robin reported it, because I have never done so.
(I acknowledge that CTRL-clicking works correctly. I think you must have misread what I wrote.)
I urge anyone reading this thread to recreate the problems mentioned here for yourselves and report them. I will email the sample tree I made with 10,000 media items if you'd like to use that. Just msg me your address.