Users of https://chromestatus.com/feature/5680189201711104 found that the API as specified was not ergonomic for subsequent offer/answer. The WG has adopted a revised behavior, merged to spec in https://github.com/w3c/webrtc-extensions/pull/238, that ensures that subsequent offer/answer does not permute the header extensions negotiated unless the user wants that to happen.
During development of the L4S congestion control feature, which is the first time we've tried to use this API to turn off a feature, it was discovered that the current definition was very unergonomic for this case. The change in https://github.com/w3c/webrtc-extensions/pull/238 is believed to not change the behavior for current users, but will make the behavior much more ergonomic for this particular use case.