When psychic terrain is active, it negates priority moves. However, it is not supposed to negate pursuit priority. On showdown, if psychic terrain is active on the field and Player A uses pursuit on Player B who is switching out a Pokémon, Pursuit still hits first. In Pixelmon If the same situation happens, it says “But it failed” when Player A tries to use Pursuit on Player B when they are switching out

Thanks for the report. You're right that this shouldn't be the case and I was able to confirm it happens so I'll move this to our internal tracker to be fixed.