BM unfortunately doesn't have a way to enforce the oath of fealty. The worst you can do is asking your judge to ban the noble but like many other things in this world, supplies and demands kick in there as well. There are a lot of available estates while only a handful of nobles to fill them which empowers nobles. Also no one is sane enough to ban a noble over an oath since it is always better to keep nobles than let them go because they changed estates. To simply put, BM is in the state where loyalty means little.
Consequences deterred real life nobles from breaking their oaths too often and oath breakers were frowned upon and considered untrustworthy by many. Unfortunately when there are hardly anyone who cares about that, no treaty will prevent people from doing so.
BM is just not a medieval simulator some people are looking for.
