yeah sorry about that, thought i had fixed this. Will be fixed with the next update (finally)
All of you are playing way faster than I expected :)
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.