Undiscovered Paradise and Land Tax interaction not working

Land tax is not triggering when this happens: I have equal numbers of lands as my opponent on his EOT, one of which is an undiscovered paradise. I tap paradise, which reads "Add one mana of any color. During your next untap step, as you untap your permanents, return Undiscovered Paradise to its owner's hand." on my untap, paradise returns to my hand, and land tax fails to trigger in my upkeep. I know paradise is returning at the proper time, because of the interaction between paradise and stasis (it stays in play, tapped), so I suspect either land tax, or just the entire "beginning of turn check for triggers" is not working correctly.

Start by reproducing the Undiscovered Paradise and Land Tax interaction described in the issue, then trace the beginning-of-turn trigger check and the implementations of both cards. Verify the return timing and whether Land Tax evaluates the battlefield state at upkeep. Done means the reported interaction behaves correctly and the relevant regression is covered by a test.

Written by the indexing model from the issue text.