Thank you for finding what I could not.

Your previous message, referenced above, reporting a problem that remains in 8.6.11 on linux, was ...

"For me, this is not limited to special characters. Trying to load anything in Tk using the 'JoyPixels' font crashes (sometimes it does load but all characters are very random - most are whitespace - and it crashes again after a call to `fc-cache`). IDLE crashes when trying to preview the font.

I believe this is what is being experienced on https://askubuntu.com/questions/1236488/x-error-of-failed-request-badlength-poly-request-too-large-or-internal-xlib-le because they are not using any special characters yet are reporting the same problem."

JoyPixels is an emoji font.  There are emoji in the BMP, but not in IDLE's initial font sample.  Did you add some, or did problems occur without displaying any emoji?
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I presume that the Phaistos 'font' has glyphs for the unicode phaistos block in the first astral plane, U+101D0..101FD.
https://en.wikipedia.org/wiki/Phaistos_Disc_(Unicode_block)

If they include one of those characters in the font name displayed in the font list (ugh), then a problem is understandable, and the fix would be to filter astral chars (and possibly more) out of font names.  But I won't assume this.

Please try the experiments I outlined if you are so inclined.