The text of true polymorph says (emphasis mine):
Creature into Creature. If you turn a creature into another kind of creature, the new form can be any kind you choose whose challenge
rating is equal to or less than the target's (or its level, if the
target doesn't have a challenge rating). The target's game
statistics, including mental ability scores, are replaced by the
statistics of the new form. It retains its alignment and
personality.
Regarding Intelligence checks, the PHB states (emphasis mine):
An Intelligence check comes into play when you need to draw on logic, education, memory, or deductive reasoning.
This indicates to me that memory is directly tied to your Intelligence and a polymorphed creature could potentially lose memory if their Intelligence changed, but later in the description the spell specifically calls out memory loss (emphasis mine):
Creature into Object. If you turn a creature into an object, it
transforms along with whatever it is wearing and carrying into that
form. The creature's statistics become those of the object, and the
creature has no memory of time spent in this form, after the spell
ends and it returns to its normal form.
Since memory is specifically called out in part of the text it makes me think that memories should be retained unless specifically stated.
Specifically, if a red dragon were true polymorphed into a human, would they retain their memory of being a dragon? Likewise, would they retain the memory of being a human once the spell ends and they revert back into a dragon?
Best Answer
They retain their memory in this case.
Going by the principle that spells only do what they say, there is nothing in the Creature to Creature function of the spell that mentions memory loss. Therefore, it's safe to assume that memory is not affected.
You emphasized:
This does not mean the creature loses their memory. They simply gain (or lose) the mental abilities of the new form they take. If a bugbear were polymorphed into a dragon, it gains the new mental prowess afforded to it by its larger, more developed brain, but does not lose any memory of its previous form (though how exactly it processes these memories would be up to the DM).
Of course, if it loses an extreme amount of intelligence, like a dragon being polymorphed into a rat (Int 16 -> Int 2), you could rule its memories would be distorted while in this form due to the fact that it can no longer process this complex information. However, this would be up the DM, as there is no rule for this as far as I'm aware.
As well, the fact that the Creature to Object portion of the spell specifically mentions the loss of memory would say to me that if they also intended Creature to Creature to affect memory, they would have said so.