Resolume stores all thumbnails in a cache for re-use. So previously imported clips will still have an upside down thumbnail, even after upgrading to 4.1.11.
All you need to do is to clean the thumbnail cache. Just delete all the files in:
Mac: /Users/[username]/Library/Application Support/Resolume Avenue 4/
PC: C:\Users\[username]\AppData\Roaming\Resolume Avenue 4\
This will force Resolume to rebuild your thumbnails right side up.