Spent this morning debugging an issue with my custom K2_Node after upgrading to 5.5, thought I'd share here in case anyone else hits it or in case I've misunderstood my own fix!
Specifically, my custom node is programmatically creating a CallFunction node calling the Map_Find function. The "Key" pin is a wildcard node which, in my case, needs to be given an FName for my map lookup. In 5.4 I was able to just set a DefaultValue (which is a string), and everything would work correctly. In 5.5 this stopped working until I manually set Pin->PinType.PinCategory = UEdGraphSchema_K2::PC_Name.
I suppose 5.5 added extra checking around whether the key pin's type is even valid for the map it was being passed before checking if it could find the key? Anyway this was annoying to debug and I couldn't find the change documented anywhere!