hidden page in master document - red arrow before

Hello,

I have a master document. Seemingly randomly, in the middle of one of my subdocuments, there is a hidden page. The page shows up fine when the subdocument is viewed on its own, but as part of the master document it’s simply not there. The page numbers don’t jump. On the page above the missing one there is a small red arrow in the bottom right hand corner.

Any idea how to get this page to show?