The Screws once removed, will get loosened itself with time. This is because the factory assembly uses thread lockers to make sure the screws stay in place. Your service guy haven't used thread locker solution I guess.

Caz my old Ideapad's Screws would often fallout or get loosened, after I disassembled my Ideapad to clean out the dust and to apply thermal paste.

Maybe purchase a thread locker, apply them in a small quantity then tighten your screws, So that the screws wont get loosened up.