It's probably sensitive to exactly how the file is opened. SMR drives need the file to be append only. If Explorer isn't communicating to the OS that it's going to do that properly for whatever reason, the driver would kick the writes back to the random access area which would slow it down.

SMR drives aren't designed for having data shuttled between areas like that. They're meant to be used such that you write in long streams directly to the shingled areas. The slowdowns are clearly due to the abstraction mismatch getting in the way.