Apple has released iMovie 10.1.6, improving stability when updating libraries created with earlier versions of iMovie and addressing an issue that could lower the volume of clips after transitions. Note that Apple recently made current iLife and iWork apps entirely free for all users (including iMovie; see “iLife and iWork Apps Now Free for Everyone,” 20 April 2017). (Free from the Mac App Store, 2.15 GB, 10.11.2+)
This is the same sort of a secret update from Apple as the new release of iTunes. Nothing new is released.
Seems, that Mac AppStore has a problem, on 28 May there were 4 updates including iMowie with a release day 25 May. None of them was possible to install. By early afternoon local time they all disappeared.
The problem is with the MacAppStores that had currency changed recently. Restart should help.