It’s a sort of error. It’s always been like that.
The GEDCOM spec does allow for both a note and a source, but the way the kiwitrees (and PGV before) code works I haven’t yet found a practical way to include them.
For the same reason there is no easy way to remove the option to enter the note either.
All unsatisfactory, and hopefully to be fixed one day.
So it is “as designed” but the design should be changed.