Hi Wind, thanks a lot for the report, and sorry for the trouble!
I looked into it, and Combat Celebrant itself was working fine. The problem was the question Neo Forge asks when it attacks. It showed the card unselected, with a title that read as if the exert had already happened, next to a big lit-up Accept button. Pressing Accept without first clicking the card actually meant "don't exert". So the Celebrant was never exerted: no trigger, no untap and no extra combat. That was the confusing part.
It's fixed now. When a creature can exert (or enlist) as it attacks, you get a simple yes/no question with the card in front of you and two buttons: Exert / Don't exert. I tested it with Combat Celebrant: pick Exert and your other creatures untap right away, and you get the additional combat phase after this one.
The fix will be in version 3.9, out tomorrow. Until then, the workaround is to click the Celebrant in that dialog before pressing Accept.
Thanks again, reports like this make the game better for everyone. 🙂