I have not found any restrictions on subtyping symmeritic references, which implies that you can have a asymmetric reference as subtype.
We already do this with abstract symmetric ReferenceTypes.

The behaviour of symmetric and asymmertic ReferenceTypes is different when browsing them, one is always handled as forward direction, for the other there is a forward and inverse direction.

That implies, that if someone does not use the symmetric ReferenceType directly, but a subtype, the behavoiur is different. Is this expected behaviour, and if yes, should we write this explicitly into the spec?

The same problem occures when subtyping a asymmetric reference with a symmetric one. When you try to follow the inverse direction, you will not get the reference since the symmetric one does not have an inverse one.