Geckolib seems to expect all custom instruction keyframes to be defined in the animation jsons as lists, and will crash otherwise. Here is the crash log: https://pastebin.com/qzRcn02N

Blockbench saves single-instruction keyframes as singletons, however:

This can be easily circumvented by manually placing each script into a list. But animation files are edited and saved over in Blockbench frequently, requiring these corrections to be reapplied each time.

Yeah I've known about this for awhile and never fixed it, gonna push out a fix soon.