It's definitely not a myth; jacket separation happens. Does it happen every time you shoot light loads in a 1:7?
What are you calling a "light" load? I have seen this argument used to suggest 55gr is a bad choice for 1:7 twists but in tens of thousands of rounds downrange, I've never seen any 55gr come apart due to twist rate. I've read people report problems with 36gr varmint grenades, which are really questionable for defense or large mammals.
It seems to me that any round that has a realistic probability of spinning apart in a 1:7 .223 is probably not a good choice for self-defense out of any twist rate.