I am using Pixelmon Reforged version 6.3.4 via the Technic Launcher modpack.
Forge version: 14.23.4.2752
Minecraft Version1.12.2
Minecraft Coder Pack: 9.42
Forge Mod Loader: 8.0.99.99
When riding Ho-Oh at any size, the player sinks through the pokemon and appears underneath while flying. Everything is normal when on land. Tested this on every growth of Ho-Oh and it still happens, albiet to a lesser degree because of size differences. Tested on both singleplayer creative and survival.
I have screenshots saved, but not sure how to attach them to this ticket.