My current engagement has me responsible for the architecture of a SharePoint publishing site collection that is migrating from SharePoint 2010 to 2016. It is quite a jump with many challenges. This article documents the workaround my team found to a vexing issue- after creating a brand new master page in 2016 using Design Manager, the master page did not show up as an option under Site Settings > Master Page.
The page was created in the same site collection using Design Manager. And it was published. And with the correct content type. And in the site's master page gallery. But curiously, the only differences between this new custom master page and the Seattle and Olso pages is the UI version and that it is an ASP.NET Master Page with no option to set it as a plain Master Page.
Attempts to set the page as 15 or 4 compatible fail because master pages are not intended to ever go backwards in the SharePoint evolution cycle. Though I am sure at least one customer has asked Microsoft about performing a reverse migration/downgrade. Any attempt to set a v16 master page as v15 compatible, let along attempting to use it on a version of SharePoint prior to 2016 will lead to the Sorry Error.
So how did we work around this issue? A shout out goes to Nicole who suggested using SharePoint Designer to set the custom master page as the default master page. It not only worked like a charm, but also provided useful information the web UI lacked.
The master page you have chosen is intended for sites with a version 16 user interface. This site's user interface is currently set to 15. Would you like to change it to version 16?
Selecting Yes and taking the user interface up to 16 allowed use of the new 2016 custom master page.