The main things that cause loss of GPS 'lock' are the satellite signals being blocked (buildings, terrain, people, etc in the way), and multipath (signals being reflected off buildings, terrain, etc). A munition a few thousand feet above the ground has none of those things.

(There's also 'GPS jamming', which is totally a thing in war, but that's not really 'like our phones', generally.)

As other posts have mentioned, though, GPS is simply one input into weapons systems, not the input into weapons systems.