Sounds to me like the baffle is loose. You can leave it as is. It seems like the newer ST´s have this problem with the muffler. Better yet, go buy an aftermarket muffler and you will be happier. With my 2 ST´s I have had nothing but trouble with the mufflers.

As far as the retaining screw one of my ST90s doesnt have one bc it came off in flight. I have seen any noticable difference in performance. You could JB weld it in place without any problems.

With the ST90s I have seen they seem to be pretty sensitive about tank location. Of all the ones that I have seen that wouldnt stay running it had to do with the tank being too high.

Also my 2 st90s seem to be a lot happier on good ol´ 5%. I tried 15% and didnt notice a big jump in RPM but did have more trouble getting them to run as well.