Board Thread:General Discussion/@comment-32182236-20190716003715/@comment-32182236-20190818210122

The Undying fact is quite useful, though.

You get your turn against her.. Even though you never ended the fight, and supposedly used up your turn to fight Monster Kid.

Unless Frisk had a way to absorb ambient magic from Undyne's transformation (It's determination that transformed her, not magic, so I find this implausible), it's safe to assume that Sans is the exception to the general "rule" here, not Jerry. It might make sense for Jerry to give you the turn (maybe that's why the monsters don't like him), but Undyne? No way!