Tools menu -> Options… -> Libreoffice Calc / View tab -> Visual Aids section -> “Page breaks” checkbox
I run in to this on occasion, and here the solution. Sometimes upon opening a file, and certainly after printing, my spreadsheet will retain the additional dashed lines that show how many cells will fit on a printed page. These additional lines annoy me, so I wanted to know how to turn them off. After some Internet searching, I found the answer.