At a note right click -> Preferences -> change the default "Title font" to something other (font or-and size) -> OK. The default font never changes. v.4:16.04.3-4~deb9u1 on Devuan ASCII x64 KDE v.5.8.6.
Seems like the same problem applies the the note's text (message body). Can't be customized nor changed to rich text.
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No. Seems like this bug, went away. I think, we can close this report now. V. knotes 5.15.3 (20.08.3). (At Devuan stable).