I was curious so I tried with Lora, Merriwether, Literata, IBM Plex Serif and other custom fonts and the boldness setting works well for me.
The book I tried is a KFX. I am on the latest firmware, on a PW3. I see You have a Oasis but I don't know if it is important. But I am not skilled enough to answer to Your question.
Some information could help somebody to understand Your issue: on which firmware are You? Which font are You using? And the issue happens with KFX or AZW3 or both?
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