Unnecessary showing admin content in main theme In one of my sites I have the bug: when I submit some admin forms, for example after editing a view, or create a taxonomy vocabulary the result page appears in the main site theme instead of admin theme.
If I simply visit these admin pages, for example /admin/structure/taxonomy they appears in admin theme as designed. Also these admin pages is showing in admin theme if i refresh them after form submitting.

So the front-end theme appears only after form submitting on only some cases. For example taxonomy vocabulary creation has this bug, but taxonomy term creation does not have it.

No themekey or some similar modules installed.

What can be a cause?
If you use customError module, make sure you your theme dropdown setting is set to "Default theme" in yoursite.com/admin/config/system/customerror.