The fact remains that you imported the polygon without it matching the adjacent one. 
You imported stream fragments that don't connect to anything, instead of importing a complete watershed. 

Looking around a little more, your edits to existing polygons may even created invalid multipolygons in adjacent tiles. (See relation 16665336)

It could be argued your mechanical edits are making the map worse in this area.  I'm trying to give you the benefit of the doubt, as you have a very high edit count ... but this means you should know better. If this were any newer an account, I would honestly be reverting your changesets.  

I'm hoping you'll come back and fix these issues, but it looks like you have moved on and are importing in several other areas now. 