Clearly jumping into the middle of a much longer conversation, but re: mysteries in TTRPGs...
I do, personally, agree that for something to be a...let's call it "sincere mystery" rather than a "true mystery" since that "true" bit there is causing a lot of issues...
Anyway, for something to be a "sincere mystery" to the players, there has to be:
1. An unanswered question that the players really want answered (such as who the perpetrator of a crime was, where a missing person has gone, or what chain of events occurred to result in the observed current situation)
2. A truth-of-the-matter within the fiction: that is, there is a particular answer within the fiction that was true from the very beginning, rather than being authored due to player actions after this specific story begins
3. In-fiction veritable evidence, which may include false leads or intentional deceptions from NPCs, but necessarily must also include evidence which points to the aforementioned truth-of-the-matter consistently, regardless of what actions the players do or do not take
If any of these elements isn't present, then it isn't a mystery to the players. Sort of like how a mystery novel cannot meaningfully be a mystery to the author, because in order for the author to write an effective and compelling mystery narrative for others to read, they have to know too many things for #1 to be true; they already know the answer to the unanswered question.
In the context of a TTRPG, you can have it be the case that there is a genuine unanswered question, but fail to meet one of the other two, which is unlike (most) novels, where it's expected that there be a truth-of-the-matter in most cases and the whole point is to provide a mixture of evidence that the reader must suss out. That is, you can have the group be acting out a mystery from their characters' perspective, but to the players it isn't actually a mystery anymore, it's the group collectively acting as authors for a mystery someone else could experience by reading through a written-down description of what they did. Or, there could be a fact of the matter...but that fact of the matter could be retconned (usually a GM action, but could be a PC action in some games), a form of non-mechanical fudging, in order to heighten tension or the like, but this undercuts the players-solving-a-mystery process because they can't actually understand or analyze the evidence, because what was genuinely reliable and valid evidence one moment becomes false, invalid evidence without any way for them to know this happened.
More or less, in an absolutely rigidly "no-myth" game, I'm saying you can't truly solve a mystery as a player, just as (for example) the character playing Sherlock Holmes in an adaptation of (say) The Hound of the Baskervilles cannot himself be "solving the mystery of the Hound of the Baskervilles" because what he's doing isn't mystery-solving, it's "portraying a character who is trying to solve a mystery".
It's why I don't run a rigidly "no-myth" game with my DW group--and why I don't think DW (or, indeed, most PbtA games, nor most adjacent games like FitD stuff) actually are completely 100% "no-myth" systems. There can be a little myth, but if there is, it needs to be serving a specific and valid purpose; one of those purposes is to craft a mystery-solving experience where the players are not just "portraying a detective doing a mystery-solve", but actually solving a mystery themselves in the process, just as most mystery-solving experiences are about solving a fictional mystery established by someone else (unless it's your job to solve IRL mysteries, of course.)