SOLR-8131 makes managed schema the default in Solr 6.0. So the "add field" & etc buttons aren't being shown in the schema link as they are in 5.x (5.4+).
Note, all the configsets in 5.5+ are managed, as they are in 6.x The difference is that you do not need to specify anything special in 6x.
So whatever the key that determines whether the admin UI knows the schema is managed or not probably needs to be updated.
Upayavira any hints here?