Odd ebook formatting behaviour with epubs

[bandit]

Hello, everyone. I don't know if anyone can help with this, but I'm gonna ask anyway.

When I import, convert, update metadata, or otherwise just breathe on an epub file lately that I have... um... acquired from the interwebs I get a very strange behaviour from Calibre. The first three, four, five, or six letters in a word get shown in BOLD. The rest of the letters in the word then are displayed normally. This is displayed like this regardless of device I open the book in. I've tried it on my Kindle Fire HD 10 (9th gen), Kindle Fire HD 10 Plus (11th gen), Various generations of Paperwhites I own, my iPhone with the Kindle iOS app, and viewing it in Calibre in the built-in ebook viewer.

This is my process:

Add book to Calibre
Download/update metadata, along with the best resolution cover I can find
Save to disk folder (still as an epub)
Send to Kindle (using Amazon's app)
After this, I usually batch convert a bunch of books to KFX overnight since they run so slowly (not every time, though)

Sometimes, I will upload the original file I downloaded before I imported it to Calibre via Send to Kindle app if I've checked it's metadata and discovered it to be complete as-is.

This makes the books quite hard to read, obviously. If you wish, I can upload an epub that displays the behaviour, but that may be against the forum's rules. You can PM me for a work-around if that is agreeable.

My particulars:

Computer running Calibre is an iMac (21.5-inch, Late 2013) running MacOS Catalina 10.15.7
Calibre version is 6.11 (most recent compatible with Catalina)

Thanks in advance for any assistance you can provide!