iNaturalist simplifies complex borders in order to keep the site usable. If we used complex boundaries for borders at follow rivers, as in this case, it would make the site too slow to use. So here is the boundary for Nambia in the Rundu area:
You can see it doesn’t follow the river too precisely, thus some observations made on the Namibia side will end up in Angola, at least according to iNat.
What you can do is put the point of your observation within the Namibia boundary and increase the accuracy circle around it to encompass the actual location. Not ideal, but it should work.