Hello there,
I've investigated the issue, and indeed, there was a minor bug affecting the chance of unlocking new animations, particularly if you were killed by a muscly. However, it's still possible to unlock others, albeit with some difficulty. In your case, it might be a combination of bad rng + this bug. Fortunately, it's already been addressed in the upcoming public version. While I could roll out a fix, but it will take too much time for a small bug (compiling took hours).
Thank you for your patience.