Okay, still the same issue with the 3.1 C# API source code.
What I'm trying to say here is because Playcount is a file-property type and not a Metadata type, you can't change it from a Plugin unless you use PlayStatisticType.IncreasePlaycount, which only increases playcount by 1. You can't decrease the playcount or reset it.
reference:https://getmusicbee.com/forum/index.php?topic=22010.msg129205#msg129205
I know you can reset the playcount using tagging tools inside of Musicbee but you can't seem to do it from a plugin. There just isn't a method inside of the Musicbee Plugin API (Class/interface/structure) that allows you to change fileproperties.