Bug in 3.38 for TB 12.0.1 Rated 3 out of 5 stars
There is problem with new version (3.38) and TB 12.0.1
Here the bar in version 3.37 (No problems):
Here the bar in version 3.38 (Curves icons)
Use small icons - Enabled
I'm unable to reproduce the bug you observed. For me, it works correctly, as expected, and with several OS : windows XP, 7, Linux and with all icons configurations : icons+text, icons alone, text alone, small or big icons.
Sometimes, I observe that after a theme update, there are a few things wrongs in the interface, but things are good again after a TB restart. Why ? I don't know.