Hi,
Here are the steps to reproduce this font.
1. Type some text in a note file using a font other than "Default" e.g. Arial. Somewhere in the sentence include a hyperlink (either automatic hyperlink to a Scripture reference or insert another hyperlink).
2. Select some non-hyperlinked text in that sentence and change the font to default. It works!
3. Select just the hyper-linked text, or the hyper-linked text plus some normal text, and try to change the font to default. It doesn't work!
This only applies when you try to change the font to "Default." You can select hyperlinked text and change it to any other font in the font list and it works as expected.
I don't know if this bug has always existed or whether it is a new one. I noticed it for the first time this evening. It's not a big issue, but it would be nice bit of polish if this was fixed before 4.5 is released.