After I save a Libre Office writer docx with my default font Arial, it very often, but not always, opens next time in Liberation Mono which is very annoying… I have set all my defaults in writer to Arial using Tools | Options | LOWriter | Basic Fonts but it is still happening. Does anyone know why? Thanks.
Edited by @Hrbrgr from Comment:
Which OS? Which LibreOffice version? How was the DOCX created (e.g., using shell’s New menu)?
Default font has no effect on the existing documents, only on the newly created documents (and only when you don’t use a template).
I’m using Windows 11 64 bit, Libre Ofiice version 7.5.1.2. Sorry I don’t know what Shells new menu is, but you may have hit the answer anyway. I create a new writer document but have been copying and pasting text from a particular website (Lyrics and Chords to songs on ultimateguitar.com). Perhaps they have been created with a template which, as you say, would have no effect?
Paste your text as unformatted.
Press Ctrl+Shift+V and choose Unformatted text from the dialog.
Then you can apply styles to the pasted text. For normal text, you should use the “Text Body” paragraph style.
You may want to know how style sheets are applied:
Professional text composition with Writer
English documentation
Thank you, that seems to have done it. 
When you use the context menu of Windows-explorer to create a new file, the concerned software (LibreOffice) is not started. Windows just copies an existing “empty” file. But as this file can have its own settings, this ignores all default settings you make IN LibreOffice.
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The folder and settings are kept in Windows under ShellNew in the registry.