stargazer_dragon
Explorer
The best argument for Polymorph and true Polymorph is where the Temp HP section is located.The problem with spells is that this distinction is not explicitly made, so people are poking holes at the rules to see what sticks. WotC assumes that players will follow the Concentration rules since it says spell effects end when concentration ends, but because spells like Polymorph don't have that specific line that the Barbarian feature has, then this must mean that they follow standard THP rules. We will continue to have this debate until errata comes out because there will be players who will stick with their conclusions until official ruling comes out.
There are only 4 spells in the 2024 PHB that grant THP and have Concentration: Polymorph, True Polymorph, Shapechange and Heroism. Of those, the only Heroism specifies that its effect ends with the duration.
Because, it is located in the section that specifically details out effects of the shape shift, and since all other detailed effects of the shape shift end with the shape shift. (though it really should have been called out more clearly)
There is a solid argument that the Temp HP end when the other effects and and that if the Temp HP not have a description of when it ends, the other effects lack of a description must also mean they remain.
Also side note, that concentration section is largely being taken out of context. already made a post on that though, but I would urge you to read it more carefully.
That said shapechange is a different beast, but it has more issues then just when they go away, and is a 9th level spell so meh.