The part of the roundabout within Germany is outside the Dutch built-up area and unlike Avantis there is no German built-up area here. There are no signs for entering/exiting the Dutch built-up area here, but that is irrelevant, because Dutch built-up area signs are invalid under German law (wrong colour). The national border is also in the correct place (verified with government maps); it has no signs here, but signs are irrelevant for national jurisdiction. Hence zone:traffic=NL:urban on the Dutch part and zone:traffic=DE:rural on the German part, both with default speed limits.

Looking at historical maps, it turns out that the road was originally built on the Dutch side of the border, and that the roundabout was inserted later.

Of course this is a theoretical exercise; in practice neither 50 km/h nor 100 km/h is a feasible driving speed on the roundabout.