'[MS-ADTS] 188.8.131.52.1 Schema NC' says that schemaInfo should also be bumped when modifications are made. We currently only bump it when we add schema classes or schema attributes. In the case of the samba-tool schemaupgrade code, we will be bumping less than it probably should.
I tested this against Windows, and modifying searchFlags, for instance, bumped the schema version. Modifying the dMD didn't bump it, it seemed.