When you create a Maven 2 hosted or proxy repository in Nexus 3, a version policy of Mixed is allowed. Once created you cannot change the policy. Nexus 2 never allowed Mixed version policy on hosted or proxy repos.
There is no reason that I can think of that we should allow a creation of a "Mixed" version policy repository.
A Maven repository should not contain a mixture of SNAPSHOT and release versions.
The only exception is a group repository - a group repository does not "contain" assets other than metadata about its members for performance reasons - a group is implicitly mixed version policy to allow mixing snapshot and release versioned group members.
There are tasks that depend on version policy - I expect tasks won't always handle properly a repo that contains both types of versions.
Consider adding a health check that detects Mixed version policy repos.