you must have gone into a "disputed succession" situation, where the country with a RM with you and the highest prestige will get someone from their dynasty as an heir.
it doesn't work that way.
1. it would be highest relative development that decides which RM partner delivers new noble, not prestige (you likely read old version of my RM/PU guide only

) You can get noble ruler of new dynasty, but in that case he will take throne directly, and be 21 years old.
2. Austria likely had 'marriage policies pay off' event, which installed heir of their dynasty in your nation.