Dyneema, as line goes, is better than most at resisting chafe. You wanted the soft shackle as the metal shackle was bothersomely noisy at anchor. I suspect that means you have a roller furled main with a little sail emerging that causes the movement.
In any case the movement experienced by the metal shackle was probably enough to cause the Dyneema to chafe over time. No line likes constant movement and slapping about. The other candidate would be UV damage, but that seems unlikely.
If you think this is the case, I would suggest a lashing, seized at both ends. Lashings give lots of warning that they are starting to chafe or have problems and have other benefits over soft shackles, especially in areas where frequent release is not anticipated.
As an aside, I think high modulus lines are great but many are finding uses for them on cruising boats that lead to unintended consequences, sometimes serious.
My best, Dick Stevenson, s/v Alchemy