For the coins of Rwanda-Burundi (Belgian Congo, Rwanda and Urundi)

The spelling of the top-level name  (Belgian Congo, Rwanda and Urundi) misses the letter B (=> Burundi) - the name on the coin level is correct (Rwanda-Burundi)

Not really, Ruanda-Urundi is the initial territory, first part of German East Africa, then administered by the Belgians before it became two independent states Rwanda and Burundi. I think the current arrangement makes sense.

https://en.m.wikipedia.org/wiki/Ruanda-Urundi

It is nevertheless irritating, when the only coin says “Burundi” and the lower level name  is also “Rwanda-Burundi”

Still, the issuer by Numista definitions is Ruanda-Urundi. The Belgians only renamed it in 1961, but they didn't split up into two independent countries (and hence issuers) until 1964. Before the Rwanda-Burundi Franc the Belgian Congo Franc was the relevant currency, hence also the grouping of the two under Belgian Congo,  Ruanda-Urundi, I suppose.