I found a bug when I do the v1 strike. The rockets sometimes crashing on mountains in some maps with mountains or high buildings.

Is it normal or a bug is my question..

This is a well-known issue for the V1 rockets. Though probably not strictly a bug it could definitely be called an issue IMO. It's up to the devvers to make do something about it.

The V1 rockets have a pretty low altitude and will explode on first collision. I often don't even see more than say, 4 rockets, actually reaching the target.
As Chyros said, it's a pretty well known issue, not sure if the developers plan to change it as V1 are pretty much unguided so it's a standard kind of behaviour for that kind of weapon.

So v1 rockets are just useless rockets?

That depends on starting locations and maps. Even in mountainous maps they can be deadly if your starting location -> target is a relatively straight line with no significant objects in the way, all the missiles could do their damage; even in a flatter map they can be useless if your target area have mountains or high buildings surrounding them.

The V1s are not useless, either. The nerve gas released on impact can kill quite some guys, and the impact damage is not bad either. They are old, yes, but depending on your point of view, you may love it or hate it.

General Alexander's Hypersonic Cruise Missile also has the same issue as the V1 rocket barrage.
*remembers first post in E-Studios (now Fallout Studios) inside Foxhound's V1 problem thread*