MC 1.20.1, Fabric API 0.89.3+1.20.1, Deep Mob Learning: Refabricated 0.5.14-BETA+1.20.1. Let me know if you want more mod versions.

I had issues getting an arena to register when I first tried this mod, so I turned the registration off. I then got a set of glitch armor and modules and equipped it. From this issue (#140) someone suggested I make the arena square instead of circle like the wiki says, so I did, and the arena was recognized. However, when I put my old equipment back on, it stopped being recognized again.

I tried both diamond and netherite pants and chests, with various enchantments. If they are unenchanted then the arena will be recognized. If they are enchanted at all, even with protection 1, then my character will be emitting flames and the arena will not be recognized, and trials will not start.

To reproduce:

1. Create a valid arena
2. Use a key on the keystone and ensure it works
3. Put on a piece of enchanted equipment. Any enchant, any number of enchants, probably any slot (I only tested chest and legs).
4. Verify that the keystone will not work, that your position is marked as the invalid spot, and that your character emits the fire particles.

Edit; Additionally, enchanted armor prevents the trial from starting. But if you take it off and start the trial, you can put the armor back on and the trial continues as normal...