rules for joining spines

[jthickstun]

I'm curious about the rules by which two **kern spines can be joined (*v). The javascript verovio validator throws an error if two spines with different time states are joined:

"Note durations do not match for designated join spines" (line 279 here).

This rule makes sense to me, but there are some humdrum scores that do not conform to this rule. For example, at line 362 of Chopin's Mazurka 33, No 4.

Are examples like the one above errors? If they are not errors, is there a rule for determining the time state of the resulting spine after the join?