Attaching any item to a soldier whilst on the ground is fine, although slightly delayed to actually attach (may be part of the issue). However any attached objects (ie chemlights, demo charges) that are attached using attachTo will fly off as soon as you open your parachute. These cannot be added back on during descent as they all fail to "stick" to the player and can be seen as static objects where they spawned in.
This has been tested repeatedly over 40 times today with :
a) descent from 50000m and repeated testing with debug console
b) mission restarts
c) sqf code