Both are general rules.
Seriously, though, in such a case, IMO it should have been called out in the spell that the temp HP go away when concentration is lost. I am sure (like others) that was the intent.
Going off of the other thread, does anyone
KNOW if a Druid assumes While Shape, gains temp HP, and then leaves Wild Shape early as a bonus action, do the temp HP go away??? It doesn't appear so to me...
If a druid keeps the temp HP after leaving Wild Shape, I see no reason why the target of Polymorph would lose the temp HP because the caster stops concentrating.