I tried modifying a layout by highlighting a column of text and choosing Format panel > Layout > Columns > 2. Didn’t like it but made a number of other edits in the interim (no Undo, in other words). Now, after setting the section to be a single column, Pages has made a hash of the line spacings where the Sections breaks were and I cannot revert.
When I view invisibles, the Section breaks are still there, and there’s no way to delete them. Any text around them can be removed, but they persist. It’s bizarre; I’ve never seen anything like this.