As far as I experienced, longbow grenades do not teleport.
If something like an enemy cuts the "warp-line" after the longbow animation is launched the grenade would end its flight at the enemy as it pops off it.
Grenade mods with the throwing type longbow / lungboom also cannot surpass fixed obstacles, even if there is an area behind where it could possibly land.
Imagine aiming at a closed door, the grenade would bounce off (or stick on it, if it features the sticky / stik accessory) but would not "beam" itself into the room behind.
It often seemed to me, that the grenade goes "through the enemy" too. But in fact I just missed them by a few inches as and some enemies, like for example rats, a very nimble and have a sleek hitbox.
So it might occur that an enemy will stop your longbow grenades flight but it might happen very rarely. It could happen more likely at bulky enemies like a Goliath, but in general, using Explosives against Goliaths is not the best idea if you are not up for a decent brawl.
Some useful extra: longbow grenades of all types go very well with the sticky accessory. This way you can attach for example a "lungboom stik sluj murrv" (I love bandit namings) to one enemy without exposing yourself and - by a bit of luck - have them slagged so you dont have to switch weapons in order to deal double damage.