Board Thread:Naruto Gaiden/@comment-25380140-20150514070725/@comment-5407586-20150519172208

Itachi mentions that the Mangekyō power faded away when the eye reverts to a normal Sharingan. So I guess that, when one uses Kotoamatsukami, the eye simply turns to base Sharingan while the Mangekyō power recharges.