Few things:

1. Mobs sometimes doesn't spawn

2. When blue flame on ground spawn, sometimes mobs just don't.

3. When blue flame is present, mobs spawn but are freezed 5-6 seconds

4. When blue flame is present, only few mobs come towards you, rest can't detect you

5. When blue flame is present, there is a mob with shield who spawn sometime away and start throwing shield to you continuously but not coming towards you.

6. When blue flame is present, some mobs that are inside the circle cast flame ( red cicrcle ) and are immune to blue flame damage.

7. Sometimes mobs don't take any damage or damage is not applied correctly to them, reason is unknown

8. Blue mini flames that needs to be hit first from behind to start moving to stone and change into physical mobs sometimes have to hit them twice.

9. Sometimes mobs for reason unknown deals huge amount of damage that near kills you with even 700DR+ while they are in front of you, instantly melting shield, its not part from any mechanics.

10. I encounter few times that moving just a bit more away from stone, mobs turns into flame instantly reseting the spot, not sure if this is an issue but I wasn't that far away.

Thats what I encoutered in more then 10h grind, also Imo they are way too tanky and AP cap set its way too high even for high end players.