I suspect you need to select all $Text (Cmd+A) and then use the Format -> style menu to reset the text. If the $TextColor for the note is set to ‘white’ or similar, all should be well.
Otherwise, set $TextColor before adding any text. The text area is an RTF space so text customisation is applied to selections and not globally to all text, once the first text has been added
Seeing the inevitable arrival of reversed out text, I should make the regular observation that this is not a plain text programming IDE. It is a rich text environment designed for writing rather than coding. I say this to align starting expectations as to how customisation of text ‘should’ work.