The problem is that Arma Reforger doesn't compensate for latency currently. This means you need to shoot ahead of your target to compensate for ping. Even though 12ms ping here isn't bad, fast moving targets like helicopters and cars require more lead. If you have higher ping, you will also see this "invulnerability" happen to infantry. I've made a ticket about this problem, but it seems to be ignored.

I agree. The cheaters (typically Soviets) exploiting the ability for finding the opposing teams main base then quickly taking over the map, are now exploiting the helicopters for base hopping around the opposing teams main base, again far more quickly taking over bases, the main base, then ending the game too quickly.

@Geez
My ticket was something completely different. I showed that choppers are hit way to OFTEN and without actually hitting them.