I'm running 3.6.4, which I installed around the same time as MacOS 26 "Tahoe", when I noticed the change. I run iTerm with
Settings > Appearance > General > Theme > Compact, and MacOS with
Settings > Accessibility > DIsplay > Increase Contrast + Reduce Transparency. It looks pretty funky:
If I turn off Increase Contrast, my tabs change shape and regain their color fill.
My guess is that there are unresolved issues either with "Liquid Glass" or with the way iTerm is interacting with LG.
Are you running Tahoe?