MSC2946 dropped a room_id field from each entry in children_state, since it is redundant. This change was not properly reflected in Synapse or Dendrite.

I'm updating complement (matrix-org/complement#422) to no longer depend on this field, but Dendrite still returns it. I'm unsure if it is as simple as removing this line from gomatrixserverlib, but it seems to be an initial spot to check at least:

https://github.com/matrix-org/gomatrixserverlib/blob/a75fa15a03a00236a26744e0fc6cea20b4eb69ee/federationtypes.go#L799

Cross-linking the Synapse issue: matrix-org/synapse#10946.