Oh wow, you're right, slecault. My main work system is Win10, and I just recently upgraded to 12.30 (because I wanted the supported Python version bump). One of my test VMs is Win10, too, and still has 12.20 installed on it. Just checked and, sure enough, Tab works to complete there. Glad I wasn't just imagining it.
While I was at it, I tested another feature that seems to have been oddly frustrating only recently and, sure enough, same thing - in 12.30 but not in 12.20, typing a parenthesis or quote will automatically generate the complementary paren / quote. This is handy in editors like e.g. Notepad++ where, if you still accidentally type the closing paren yourself out of habit, it just advances the cursor a character silently, but in TestComplete's editor, it adds an extra one, so you then have to delete it. It's also annoying when you intend to keep typing after closing the paren or quote on the same line (which is pretty frequently), since you have to arrow over to get the cursor in the right place anyways. It doesn't really save any keystrokes and adds errors more than prevents them. I wish I could find a way to turn this automatic 'helpful' behavior off, as well, but I don't see anywhere in the options to do so.