When I reopen a spreadsheet it often comes up with a large empty box fixed to the top left hand corner and extending rightwards, finishing in the middle of a column. I asked friends with no success then searched online and it found a thread on this site: https://ask.libreoffice.org/en/questi... Now this same issue was raised back in 2015 but closed the following year as "not relevant or outdated" although the final entry did not present an answer but only a work-around which included the use of MS Office, which I do not have access to. This was the work-around presented (using Libre Office alone) by Loginmyeye: Tools>Options>LibreOffice Calc>View>Objects>Drawing Objects (Hide) and as he/she admitted "I don't have an answer--just more clues".
So I have this problem repeated in Libre Office version 6.0.2.1 in 2018 and have tried this work-around but if I clear the boxes using this method, it does work but if I close the file and reopen it they are back again, so it's not a FIX.
Did you already try a new user profile thoroughly regarding all the precautions?