So maybe Primitive plus is problem here but there is alot servers that have wild wiverns onit my to, you can come to snow biome and you will see and check yt to , lots of videos of how people use code to spawn them
I tried to spawn them in and I know how to spawn them in. I spawned in rock drakes in the Floating Isles and not expect to have eggs spawn there. But they did. Ive spawned wild dinos and tame dinos. Even I use the long blueprint spawn codes. But some reason on Primitive Plus it won't spawn.