Moving between EZ directories should work fine if the EZ key for the directories is identical. If the key is name identical then no decrypt/re-encrypt is necessary.
However, the rename operation checks more than the key name. It compares the inode number (unique identifier) of the source and dest dirs which will never be the same for 2 dirs resulting in the cited failure. Note it also incorrectly compares the key version.
A related issue is if an ancestor of a EZ share the same key (ie. /projects/foo and /projects/foo/bar/blah both use same key), files also cannot be moved from the child to a parent dir, plus the child EZ cannot be removed even though it's now covered by the ancestor.