That's the thing, I did not miss it, that's why I had the Bestiary in the screenshot to show I had completed all forms. Had I not had the 65 forms I would have not gotten the Bestiary, (I tried). I finished the game twice with 100% and it seems the first time you finish it you always get 64/65. Funny enough if I finished it again I get 65/65. This had happened to me twice and it also happened to Ghast in his run.
Huh, that is weird. I hadn't realized that was a thing! Guess I'll go check that out!
Wait no, I thought I fixed this! You're playing on the latest version of the game I assume?
I wasn't sure about the version. Tooltip windows claimed 1.3.2, but I wasn't sure if that meant 3.2 update. Now I can confirm that is 3.2 since I was able to reset the fuse puzzle.
Okay! I have figured it out, maybe! Did you examine yourself to get the Wizard form? If so, that may be what's caused this. Basically, what happened with Ghast's run was that the game didn't update what forms had been collected until the game restarted, and I thought I had fixed that, but I may not have fixed it if you examine yourself. Also, did you do the game in one continuous playthrough, or did you pause and reload the save?
I did examine myself and I did played it in one sit through in both cases. So basically I replicated a weird bug XD. Interestingly I found an other bug but I couldn't replicate it:
Double Gargax: Two bosses spwan and neither of them can sent you to the previous screen. This makes it easier to finish the boss. This was actually how I finish the game the first time. I don't know how it happened nor if it's replicable, but I'm just warning you if someone finds it again.
Another weird bug that I found out is that if enter the Boss and keep pressing the mouse button, you can drag the cursor behind the wizard. If you do, so now you can enter the boss and move around to dodge him. I did replicate this one and I suspect that it might be related to how I produced the Double Gargax bug.
I know about the double boss bug, but I have no idea how to replicate it so it's still in there. I will try to fix the moving around bug though! That's not supposed to happen, ideally. But, on the bright side, I've now tested my fix for the forms issue and it seems to have been dealt with! Thank you for bringing it to my attention!