The party sorcerer polymorphed himself into a T-Rex and got his CHOMP-CHOMP on with the Mummy Lord, taking a few hits in the process. The T-Rex now has mummy rot which is described as:
The cursed target can't regain hit points, and its hit point maximum decreases by 10 (3d6) for every 24 hours that elapse.
There have been several questions relating to conditions, spell effects, and other happenings asking if their effects persist on a player who then polymorphs into something, but I haven't found anything related to what happens when it's the other way around.
If this were a condition, we know that it would persist on the player after leaving T-Rex form:
A condition lasts either until it is countered ... or for a duration specified by the effect that imposed the condition.
However, mummy rot is not a condition, and none of the damage done by the mummy will carry over to the player's original form once Polymorph ends because:
When it reverts to its normal form, the creature returns to the number of hit points it had before it transformed.
If a polymorphed player gets mummy rot, does it persist when they leave their polymorphed form?