Quoting @andersonhc in #1133 (comment):

I am running some tests and it's interesting your PR makes it clear we are letting unused fonts end up in the output document.
The page 1 will have "F1" and "F4" in the resources dictionary, and page 2 will have "F3" and "F4".
"F2" is added on the final document and not referenced at all. "F4" is added to the resource list because of set_font() although not used.
I have documents with many fallback fonts and there is a considerable amount of unused font data added to the documents.

fpdf2 should NOT include unused fonts.

We should implement a mechanism to only add to the final PDF document the fonts actually used.

cf. comment #1382 (comment) below for mor details.