When system creates key for workflow process it takes all (or selected) operation parameters, but it expects all those params to be strings (or integers etc. ). To build a key string it just concats the value of this param. So in case it is an array of values it will add "Array" to this string. So if two processes are different only by this param, system will use incorrect workflow process (for example it will fetch exiting one instead of creating new).

The only operation now with array params is "updateobjectstate" and it is not enabled by default, so not sure if you are going to fix this.
Anyway here is the patch, maybe it is not very nice, but it garanties uniqueness of the key, and this main purpose of this function.

Cheers,
guys