arguably that's what it does already. if the landing spot happens to be a tank and able to move it will just move with it until it explodes. imho still a better solution that magically phasing through the vehicle onto the ground.
anyway, i think this is a pretty insignificant issue anyway given how incredibly rare it is to hit a tank with a sticky mortar round.
How long was the fuse again? For some quicker vehicles you can still evade, especially if you've already started driving out of the danger zone and the vehicle has built up some speed. but I agree, overall these issues are very marginal.
Actually it is not that hard to hit slow vehicles like Ele/JT even the Pak howitzer can do it using the HEAT rounds.
Which is exactly why it does not really matter that much for large and slow vehicles. They could rarely dodge the shell anyway. Smaller and lighter vehicles are more effected by this since also the 80 damage has a larger impact, but then again I have to agree with MMX that the overall effect of fixing this will not be large. I'd personally still rather have it phase through the vehicle and hit the ground below, but if this causes a lot of work, just leave it be. This time is better invested elsewhere.
The round does not have to "phase" it simply need not to "collide" with vehicle and simply land on the ground.
Which is exactly what phasing through the vehicle means...