After a Youkai Book morph is collected the game will crash the next time you open the Morph Menu. (I have not attempted subsequent menu opening after this starts.) Afterwards having the information in your player data will cause RAM issues until Metamorph is removed. I suspect this is caused by the mob's animation/model somehow.

Could you please post a crash log, please? And what do you mean by RAM issues? Memory leaks?

It may just be that the data has had time to clear out, but it doesn't seem to have happened in the latest version again. Maybe it was just the limitation on the number of morphs you can have and I decided to try morphing into it with bad timing?

Clearly not a limitation of how many morphs you can have, but might be something related to data dependency of that entity in the rendering code (thus crashing due to lack of data).