After upgrading a 2010 site to 2013 in 2010 UI mode, I'm unable to apply a custom master page to application (_layouts) pages. The custom master page has been configured as the Site and System master page and shows up correctly throughout the site whether logged in or not. However, I have a custom login page, e.g.
And the master page is not being applied there as it was prior to upgrading. I have tried both of these variants in the Page declaration:
It reverts to
/_layouts/simplev4.master either way.
Update: switching from having the page inherit from IdentityModelSignInPageBase to instead use UnsecuredLayoutsPageBase solved this.