I have a problem with installed fonts, even standard fonts, in 17.10 not being displayed correctly:
Below screenshot is how the fonts look in the font selecter:
Are you seeing that for all fonts, or just some, like the ones you show?
For the ones you show (I see LKUG and Lohit Assamese), that's normal. The weird symbols mean that the font doesn't have the characters needed to display those letters. The sample text is almost certainly something in Latin script, probably "The quick brown fox...", while those fonts only include characters for specific other scripts. It looks like LKLUG is a Sinhala script font (spoken/written in Sri Lanka), while Lohit Assamese is for . . . Assamese (from the NE of India).
If the sample text were written in Sinhala, then you'd get those symbols for your usual fonts, since they have Latin characters (and maybe Cyrillic and Greek), but not Sinhala. Some fonts include multiple scripts, but it's common for each script to have it's own font, so Ubuntu needs to include these fonts to support writing in those scripts.
Here I opened the app Character Map, changed the font at the top to LKLUG and then on the left column changed the Script to Sinhala:
Your explanation seem to fit with what I’m seeing.
Only some of the fonts are showing like this.
The reason I thought I had a problem with fonts in the first place was that Firefox was showing boxes instead of letters on the IBM Plex page. And then i checked the installed fonts and saw the same.
Now I realise that the two are probably unrelated. And I think I will clean up my installed fonts.