Add "Browser Log" action with the following string:
"Overheat:" & Overheat & " WeaponLevel:" & WeaponLevel & " RESULT:" & ((Overheat / 6) * (WeaponLevel - 100) ^(1.5 * ((WeaponLevel - 100) / 10) /2))
Open browser console in preview (F12), try to find a bullet with NaN damage, you'll be able to see the values of Overheat and WeaponLevel variables, maybe there is something wrong with them.
If this doesn't help, please post a small project demonstrating the issue.
.
Edit: (-10)^1.5 returns NaN, so the most likely reason is that WeaponLevel is less than 100 for some bullets. A simple fix would be adding max(WeaponLevel,100)