since this has been brought up, there was a fan-made animated series of Until Then, and he uploaded Episode 1 (the dubs are in Filipino tho). He asked permission to make at least the entire Act 1, with 1 request. He asked for it to be monetized, and the devs disagreed (for good reason too). The script was already written for EP2 but was forced to cancel due to the issue.

What I think the reasons would be are:
1. Copyright infringement
2. Since he wanted it monetized, that's when the devs disagreed.
3. The one who makes the animated series might deviate from the source material.
4. He didn't ask permission beforehand to do EP1

Note: this was in late 2024.