This looks like a regression introduced by OAK-2441. If a node is deleted and added again, the mixin call will return mixins from the previous node.
Node.getMixinNodeTypes can revive deleted node mixins (again)
Node.getMixinNodeTypes() may check for child node named jcr:mixinTypes
Inconsistency between Node.getPrimaryType and Node.isNodeType
Regression with Node.getPrimaryNodeType and getMixinNodeTypes wrt Jackrabbit 2.x