If a bucket is created and owned by a tenanted user, then an error will occur if you try to change the owner to an owner with a different tenant.
E.g. create a bucket owned by user 'test1$abc'. Try to change it to 'test2$foo'. This will raise an error. But it is possible to change the bucket owner to 'test2', 'admin', 'test1$bar', ...
Because of that the list of possible bucket owners should be filtered and only display the owners that make sense, in this case all non-tenanted users and the tenanted users that have the same tenant. The tenant can be determined from the bucket name (<TENANT>/<NAME> => test1/my_bucket).
This only applies to the bucket form in 'edit' mode.