Interesting. I put Remflex on with the new headers last year and had to pull both sides due to various reasons this year. Re-used the same gaskets with no issues. I guess I just got lucky for once.
I would have a hard time understanding why they couldn't be reused if they were you reuse with the same heads and same headers because they do make a print..
I can see if you were to change things the the mold that it makes could change and possibly leak...