Can't evolve schema after setting per-topic compatibility

[jurgispods]

Evolving schemas with the Schema Registry UI usually is a fairly simple and convenient process.

However, using version 0.9.4 together with Confluent Platform 4.0, the following steps lead to a situation where I can't remove fields from a previously created schema.

1. Create a new schema
2. Set compatibility level to NONE (my global level is BACKWARD)
3. Edit schema, remove one of the fields, click "Validate", the tooltip "You can now evolve your schema" displays as expected
4. Click "Evolve schema"

The UI seems to accept the schema evolution and doesn't display an error, but it still shows the previous schema version with the exact schema as before. Nothing changed.

I am able to evolve the schema correctly and get a new version of my schema if I just add a field.

The output of `kafka-configs --zookeeper {myzookeeper} --entity-type topics --entity-name {mytopic} --describe is empty, although the compatibility level was changed according to the UI.

Any ideas?