Currently the Mimikyu.set.json file attempts to spawn form 114 for Spirit Mimikyu, as that's the form value used for all other species "Spirit" form, and Pixelmon Spawners found in Graveyard structures also attempt to use the spec f:114

The value 114 results in a completely normal Mimikyu spawning, as contrary to other species with a Spirit special texture, Mimikyu uses the values f:2 and f:3 for Spirit form Disguised and Spirit form Busted respectfully.

This issue occurs in v8.3.6, and seems to affect both natural Mimikyu spawns, and also the forced spawns generated by Graveyard Pixelmon Spawner blocks, both using the incorrect form value of 114 when attempting to spawn a Spirit Mimikyu.

As a sidenote, this also means that the "/pokespawn Mimikyu st:Spirit" (and anything else that can use Pixelmon spec values) also fails and spawns a typical appearance Mimikyu, however "/pokespawn Mimikyu f:2" or "f:3" spawns the correct form.

Confirmed Thanks for reporting!