Thanks for your bug report. A comma has never been valid input to this identifier. It should be returning $null in this case.

It looks like adding support for year range 1601 to 9999 in v7.55 has caused it to handle invalid input differently.

I will be changing this in the next beta so that 1) $null is returned for invalid input as before, and 2) commas are allowed as separators (unless someone can see an issue with backwards compatibility here...)