I have used Word for more years than I like to remember and I use section breaks and page setup (Landscape/Portrait) frequently, and I have never come across this.
Does this odd effect only occur with this one document, or has it started occurring with all documents.
If it is just this one document it is likely a corruption in the document - I have had odd effects occur with a single document, and after recreating the document it all works fine. I copy and paste sections at a time from the old to the new document, and keep testing that its working OK. Sometimes you can isolate where the corruption is, and then just re-type that paragraph or create a new table etc.
If this is happening with all documents, I would try a Word repair, (Help - Detect and Repair ...)