noordfiets
Ah, I don't see Nominatim as a third party, but integral to OSM.
But as to a possible solution:
The current boundary for Delfzijl is the old municipal boundary. But this has been superceded by the new municipality Eemsdelta. Within Eemsdelta city's/villages are described with administrative level=10. 
One could argue that by changing the boundary for Delfzijl to the landbased portion ( as ie on G_maps ) the problem could be solved, while retaining the border between both countries as it is described by the boundary for Eemsdelta.
