If you already had K2 installed and then went to dev version then back to 2.9.0, the only difference is that when you installed the dev version, it updated some indices, but it did not change the SQL schema. The SQL schema is slightly changed only for new installations.

My guess is that something was wrong with your menus all along and it was probably a coincidence that installing different K2 versions resulted in the menus for K2 working again...