The upkeep stop doesn't work if you will lose during your draw step. Instead of letting you pause during your upkeep to take an action, it instead just automatically makes you lose. I was just playing against a mill deck using a deck with burn spells, and they milled me out, but I could have untapped and killed them during my upkeep if the stop had worked. But it didn't. So instead of winning, I lost. This is not some obscure interaction. Letting players take actions during their upkeep is a fundamental part of the game and comes up often. Please fix this NOW.