There isn't any detail in the latest release notes, but after the latest update, all arcade functions using specifically the Count(First(Geometry($feature)['paths'])) scheme for counting vertices broke and raised 'Illegal Argument' errors.
Oddly, re-witing that scheme as Count(Geometry($feature)['paths'][0]) worked totally fine...
This was also not an issue in 3.3, so something has changes with Arcade or the way arcade accesses geometry data in this update and it was undocumented.
Count(First(Geometry($feature)['paths']))
>>> Arcade error: Illegal argument
Count(Geometry($feature)['paths'][0])
>>> 10