I don't remember hearing about any issues specifically for that many upgrades, although, I am aware of the display issue when more than 2 techs upgrade the same unit. I guess I've never bothered to try past 3 upgrades, simply because I knew 3 was sufficient to cause weird behaviour.
I don't remember the details very clearly, but I suspect there is a possibility the bug you're reporting has the same roots as the display bug.
I'm particularly busy as of late, but I am rather interested in looking into this. I don't suppose you can produce a reduced example that illustrates the problem? Perhaps a tech file with just 5 techs, that can all be quickly researched (cost 1), and possibly a bare bones level template using that tech file (if needed). If the problem can be quickly reproduced without having to play a level, or be distracted by other irrelevant details, I just may find the problem too interesting to pass over, despite my busy schedule.