While playing a survival modded world on the beta 1.16 version, I decided to do a raid. The pokemon in the raid was Froakie and the Pokemon I chose to fight with was a custom pokemon. To be precise it was Discord User Blackout's datapack pokemon called Dawn Lycanroc.
As the timer hit 0 to start the raid, it kicked me out of the raid and black smoke particle effects appeared, as though I had lost the raid.
To double test this, I tried again with Lycanroc and it happened again.
So I tried using another non-custom pokemon and the raid proceeded as normal.
Therefore, I think this is a bug that disallows custom pokemon from interacting correctly with raid dens.