Unlike my previous Korean made SR705 , Indo SRFF805 and my current SRMS 725, my SR300E has notable neck relief get off easily even just little temperature or humid change.Curious what you mean by "unstable neck issue?" I do notice that I have to adjust tuning on my SR370EF every time I take it out of its bag, but it stays in tune while I'm playing it, until the next day. I wouldn't call it unstable, just not "rock solid" like my Fender Jazz and premium-level SR1350B, which do not require re-tuning every time I play them.
It's a thin neck without stabilizing rods, and only a 4 bolt neck/body connection, to a relatively soft maple body, so I guess that's par for the course?