(spoilers ahead for Rime of the Frostmaiden, read at your own risk)
While discussing monsters with changing stat blocks, the example of Auril, from the adventure Rime of the Frostmaiden, came up.
Auril has different forms, which come into play when her previous form is reduced to 0 hp. Here are the relevant texts from the first and second forms:
Divine Rejuvenation. When Auril drops to 0 hit points, her body turns to slush and melts away. Auril instantly reappears in her second form, in an unoccupied space within 60 feet of where her first form disappeared. Her initiative count doesn't change.
Divine Rejuvenation. When Auril drops to 0 hit points, her body collapses into shards of ice, whereupon Auril instantly reappears in her third form, in an unoccupied space within 60 feet of where her second form was destroyed. Her initiative count doesn't change.
It seems unclear whether those forms are considered as different monsters altogether, or if it is the same monster changing its stat block upon reaching 0 hp. The consequence of that is that it is also unclear whether ongoing effects, magical or not, may persist through a form change. It could seem logical to make a ruling that "physical" effects, such as poisons or being stunned, do not persist, but what about magical effects or mind afflictions?
The question is then the following: is there anything I missed in the text that states if those forms are separate monsters, or one and the same? How does that influences ongoing effects and their resolution when changing forms?
I'm looking either for answers based on the text as written, or for "good subjective" answers on how a group dealt with this issue in an actual session.