I've noticed this problem over several months and over the last few versions of DO, and under different file systems (NTFS, ReFS, exFAT). Apparently, there are some specific folder names where their last character will get truncated in thumbnail view, and only there:
Changing them to lower case fixes it. Changing the "RJD2" to "RJ2" will still remove the "2". A "FLC" will remain intact, however. Maybe there are internal commands similar to those and there might be more combinations that would reveal this bug.
I hope this can be replicated and I don't remember messing with any settings related to file name display in thumbnail view.