The technically inclined amongst us might be interested to hear the details of how a bug like that happens, if you don’t mind sharing. ❤️
I think I figured out how to reproduce the problem. Click the upgrade card, then quickly click the card you want to upgrade while the upgrade card is not yet in the discard, then click the upgraded card to play it while things are still moving to the discard. That seems to reliably make the end-turn button stay disabled.