text: Can't see anything wrong. (I've added a centre line to be precise, but that's a bit pedantic) I guess you are concerned that Inserting the weir split the circular "nature reserve boundary" and "waterarea boundary" into three parts.
  Then the system recombines the borders automatically into a line "relation". That is normal. Once you have a relation it is more difficult to edit, as it doesn't want you to leave orphan bits of relation behind, so you simply have to remove the relation tag whilst you edit, then replace it. (PS will be using the border relation as a quick way to create land use when I get around to it, so it will come in useful)