Have you tried using the native Odf format instead?
The Rtf format is a Microsoft proprietary format that never seemed to
manage to achieve it's promises of interoperability.
https://en.wikipedia.org/wiki/Rich_Text_Format#Criticism Apparently it even led to MS losing a court-case! MS then started to
abandon the format and their newer format, OOXML, started to appear. OOXML
makes all the same promises, apparently with a similar level of lack of
success but with a bit more care taken to avoid being taken to court.