The word “rebuilt” can mean many things on a driveshaft. To me, it means all U-joints and the center carrier bearing was replaced. Some may consider the replacement of 1 U-joint a “rebuild” job. I had forgotten some things in your original post and they raise a red flag over what has allegedly been done and what you have been told.
First point is the drive shaft being rebuilt and allegedly replaced THREE times? Anyone who would do this is doing nothing more than wild guessing; and guessing very badly.
The second point involves a question. They claimed to have replaced this driveshaft 3 times. So who in the world is paying for all of these allegedly defective driveshafts?
I hope you’re not going to tell me that the dealer claims these genuine Subaru driveshafts are defective and the parent company (distributor) Subaru of America is covering this under a parts warranty.