After a little testing in singleplayer with a platform that is only in the modified jungle biome, i had multiple river spawns like bidoof, corphish and wooper spawn on said platform. The same seems to be true for legendary pokemon, resulting in keldeo spawning in modified jungle areas instead of rivers. To make sure the platform is only in the modified junglebiome, i used voxelmap's biome overlay and a mod called "biome border viewer. Additionally i checked the edge of the platform while having the F3 screen open, looking at the biome i was in.

pictures of the platform for reference:
https://imgur.com/cRPUVrM
https://imgur.com/qB4dA9U

Hi! This ticket is now incredibly out of date, and likely no longer is relevant to our latest Pixelmon versions. We appreciate the help in reporting bugs, so if you could first download our latest version, and re-open a new ticket should this bug still occur. Please see downloads.php for the latest version!