Regression: Using a mixinType in a query template breaks the creation of a document

After our upgrade to Hippo 7.9 we the creation of a document fails in a particular case. In our project we place a mixin on a document via a document query template.
While doing this the folder workflow fails with an java.lang.UnsupportedOperationException. This only happens in case we have the mixin on the query template. While looking at the code it fails because org.hippoecm.repository.standardworkflow.ExpandingCopyHandler#startNode tries to add the mixin to a fixed list.

We do this on the query template because our document prototype comes from a different 'library', so we can't put it there.