I have a Minecraft server with the Alex's mobs mod installed one of my friends tried to tame a Hawk and it disappeared ill show you what he sent me

"Well my little birdy that took a long time to tame glitched out of existence, but I had brought enough taming materials for 2 so I tamed the other one. I made it all the way back home, stuck a name tag on it and watched it disappear before my eyes"