The monsterform will be unlocked when you get to the end of the dungeon. I think it would also transform you back into dragonform if you surrender inside the fortress.
It's not intended to revert your form and leave you in a locked state.
Viewing post in Monster Souls: Chains of Chimera comments
can I ask what you mean there?
1.) Monsterform? Wazat? I though there were only Slime, Beast, and Dragon? Is it new or secret?
2. What's not intended to revert your form and leave you in a locked state? The teleportation? It does say, in the recent bug fixes, that you worked on that in the last two?
3.) (last one, I promise), are you saying that it should fix itself if you get inside the dragon fortress and surrender rather than doing it elsewhere on the Island?