I usually use a transition piece at the corner.In place of having two pieces cut at 45 degrees to make a 90 degree corner you have the two longer pieces with a 22.5 degree cut and a small piece in the middle cut at 22.5 degrees on each side.
If it is a larger/longer bullnose you can kerf the back of the molding and wrap it.Pics would help but these are the two alternatives.
If it is a larger/longer bullnose you can kerf the back of the molding and wrap it.Pics would help but these are the two alternatives.