They put a message that they were not going to use the legacy addresses anymore, and instead P2SH addresses, that's why I created a new one, which starts with 3Ps..., and made the deposit.
I have not seen this message anywhere on the site, their social media, or marketing emails. On my account I can only generate legacy addresses for deposits. I am skeptical about this being a genuine message you received. If you can show a screenshot of this message it might be useful in determining its legitimacy.
They should have updated to wrapped segwit addresses many years ago. It could be they were doing some testing that they rolled back for now and that’s why the address doesn’t appear anymore.
No, unfortunately I didn't capture the message, it said that legacy addresses will no longer be used, and you should only deposit to P2SH addresses. Deposits to old addresses were going to charge an additional fees.
As you say at the end, I also think that it was something temporary and they turned it back.
I have not received a response from either support or TheQuin.
I hope they fix it and that address is still assigned to my account.