It's well understood that "___ Polymorph" spells overwrite pretty much everything in a stat block (or character sheet) with numerical functionality, including languages, proficiencies, and named features. Everything except "alignment and personality".
What does "personality" mean? A very inclusive option would be "any knowledge or memory that doesn't have a predefined quantified effect". A very exclusive option would be "personality traits, flaws, bonds, & ideals exactly as written, plus relationships & emotions, but otherwise tabula rasa as befitting a creature who just sprang into existence".
As pointed out, "What defines a PC's consciousness of self, memories and knowledge?" is indeed a similar question, so I'll refocus this one.
Is there rules evidence that the original creature's knowledge and memories are retained after a Polymorph, other than emotional responses about people/places/objects/actions they like or dislike (aka "personality")?
Let's look at this example creature named "Bob":
Pre-Polymorph Bob ("PRE") was an AL-legal medium-sized humanoid PC with 10 Intelligence, proficient in History, Nature, Performance, Violin, Thieves Tools, Common, Halfling, all Armor, Simple and Martial Weapons, able to cast Sorcerer spells up to 3rd level.
True Polymorphed Bob ("POST") is a medium-sized non-humanoid creature that includes the complete humanoid body plan (possibly plus wings, a tail, or other bonus parts) with 10 Intelligence, proficient in Stealth, Perception, Tinkers Tools, Common, Gnomish, Light Armor, Simple Weapons, no spellcasting.
Unlike Feeblemind or Modify Memory (the examples in the prior question's answer), Polymorph changes the creatures proficiencies. Obviously, POST is not proficient at the Charisma (Performance) or Dexterity (Violin) checks needed to put on a show or play a particularly difficult piece. But more importantly, POST also is not proficient at Intelligence (Performance) or Intelligence (Violin) checks to answer informational questions about the process of a violin performance. POST is no better at this task than an average Commoner. POST is missing some portion of PRE's knowledge. Furthermore, POST has gained knowledge (languages, skills, tools) that PRE never possessed.
If we need to use Magi-Babble to say that somehow Bob can lose and gain proficiencies without any change to their memories, then okay, I'll accept that, but I'd like to have rules justification for it. Otherwise it makes more sense to me that POST is actually a completely new being that's strongly influenced by the presence of PRE's "soul".
This link is somewhat helpful, but Jeremy Crawford only talks about "identity" and "core personality" in terms of caring about your friends, likes vs dislikes, and other emotional responses. Unfortunately he doesn't discuss memories and factual knowledge.
Pre-Bob: Hello, I'm Bob the violinist!
Alice: Oh? What do you like about violins?
Pre-Bob: Wow, let me tell you, there are so many things... blah blah blah...
(15 minutes later)
Alice: Sheesh, stop already! TRUE POLYMORPH!
(POP)
Post-Bob: I'm still Bob the violinist!
Alice: Oh? What do you like about violins?
Post-Bob: ... I don't know. I just like them, okay?