I've created two Google Docs documents in Google Workspace (Google Drive) and pasted the same text. I selected all the text and removed all of the formatting. According to Google Docs, both are Arial font 11 pt. but there's obviously a difference, one is really problematic in terms how the font is rendered with respect to spacing, etc.
The following screenshots are from the same Google Workspace (Google Drive) account, running Firefox 92.0 (64-bit), on an up-to-date Microsoft Windows 10.
The problematic Google Docs example:
But when I create another empty Google Docs document and did the same thing (paste and remove formatting), somehow I get the following:
I can reproduce this issue when I create a new Google Docs document in a folder I've created a few weeks ago. Whenever I create a new document there, I get this Arial font not rendering properly. When I try creating a new Google Docs document in a totally different folder, I don't have that Arial font problem.
I've checked the following but they don't seem to correspond to my situation exactly: