I saw a couple posts recently about the next expansion "finally having an answer/reason" as to why the Fate tokens have different sides. I am under the impression that there is already an answer, though I can't seem to find it spelled out anywhere.

The two sides seem obvious to me when you think of the different mechanisms involving fate.

1. Spending Fate. Flip a token from the "charged" side to the "spent" side to indicate your using Fate to re-roll. We use the the gold side as "charged" (or "full", "replenished", etc.)

2. Replenish Fate. Flip a token from the "spent" side to the "charged" side. This is the opposite of Spending fate. You can't acquire more Fate tokens in this manner (much like you can't Heal more than your quota of Lives.)

3. Gain Fate. Gain a new token from the bank of Fate in the "charged" state. This can increase your allottment of Fate above your starting quota.

4. Lose Fate. Lose a Fate token, surrendered to the bank. This really sucks.

If I've missed where this is clearly already spelled out in the rules, or I'm doing it completely wrong, please let me know.

"Once per die roll, a player may pay one fate token (returning it to the

stockpile) to reroll one die he has just rolled under the following
circumstances" (p. 6)

Bolded the key part. Your points 1 and 4 both are return fate token to stockpile, while 2 and 3 are take fate token from stockpile, with the difference being that replenishing cannot take you above your fate value, while gain fate has no upper limit.