I have recently upgraded to Linux Mint 20 with Libre Office 6.4.4.2. Since then duplex printing will not work with Libre Office Writer - it did with the version I was using before, with Linux Mint 19.3. It still works when printing from Google Chrome though, so it seems to be to do with LO, not the printer. The printer is a Canon MG5750. I have done everything I can find within LO and the settings of the printer to ensure it is set to duplex. The printer takes the sheet back in, as if to print on the other side, but it comes out blank on the second side and the next page is printed on a new sheet.
Please try to print from LibreOffice Safe Mode (Help -> Restart in Safe Mode). If printing in duplex mode does work in Safe Mode, please reset your user profile.
If that doesn’t work, it might be a problem with your default printer driver in Mint. The discussion here might help.
That discussion is now dated but there might be newer drivers available. I note that your printer/scanner is one of the Pixma series. Search for Pixma drivers for your Mint operating system. (I have a similar problem with my Brother printer running under Ubuntu and have to install separate drivers for it.)