rs.AddText() ignores font_style=1

It doesn’t font_style=3.

Try this:

Annotations have changed a lot in Rhino 6 and there are a few ways to create them. Unfortunately there’s seems to be something wrong with the method you’re using and were looking into it. (RH-53086)

In the meantime this also works:

from scriptcontext import doc
from Rhino.Geometry import TextEntity, Plane
from Rhino.DocObjects import Font

text = TextEntity.Create('xYz', Plane.WorldXY, doc.DimStyles.Current, False, 0, 0)
text.Font = Font.FromQuartetProperties("Courier New", True, False)
text.TextHeight = 0.1