There are plenty of comments, dating back to 2017, and this still doesn’t work. Edit default font doesn’t work, still comes back as Liberation Sans 12, save as template, still comes back as Liberation sans. In Mint 20 on LibreOffice 6.4.6.2
No problem here using Tools -> Options -> LibeOffice Writer -> Basic Fonts (Western) for new documents. Existing documents or using a customized default template won’t be affected by this. If you are using a customized default template assure to File -> Templates -> Manage Templates and click to the symbol in left bottom of the dialog and select Reset Default Template -> Text Document
(If nothing appears/happens when selecting/clicking `Reset Default Template` you don't have a customized default template)
Just tested using:
Version: 7.1.0.3 / LibreOffice Community
Build ID: f6099ecf3d29644b5008cc8f48f42f4a40986e4c
CPU threads: 8; OS: Linux 5.3; UI render: default; VCL: kf5
Locale: en-US (en_US.UTF-8); UI: en-US, Calc: threaded