the (non-normative) diagram for ice state transitions shows a transition from 'checking' to 'disconnected' (also in the opposite direction) which is not shown in in the equivalent peerconnection diagram.

That looks like a bug, I can't think of a way how the checking->disconnected transition would happen. And disconnected to checking in the case of an ice-restart would happen via new.

(note: even though both diagrams look quite different otherwise that seems ok since in the ORTC version there is an annotation that 'new' is reachable from most other states, reducing the visual clutter)