How did you set up mirrored margins?

• By using a gutter in margin settings with different even and odd headers? or
• By selecting Mirrored Margins in the margin gallery?

In addition, "mirrored margins" is a document setting. Different even and odd with a gutter is a document setting as well.

On the other hand, other margin settings are section properties. You may have deleted section breaks containing this formatting.

-- just saw your update. Glad you got it fixed.