I've had the same thing although I cannot seem to reproduce this bug every time. Sometimes the arming process works like it should and other times it doesn't (same location).
This morning I wasn't able to arm it the first time and also couldn't pick it up afterwards. When I tried it a second time, I was able to arm it but in most cases I can try as many times as I want but it won't do it and I lose my plastic explosive :(
I'm not sure if I used the same 'Remote Detonation Unit' when i succeeded the second time, maybe using another one 'fixed' it that time?

I've never seen anything like that... In fact yesterday I experienced the opposite.
I was playing on a (stable) vanilla server and even though I was almost right under de impact zone of the gas attack (so I exactly knew where to look afterwards), I couldn't find ANY PO-X Vial when the gas was gone.

This ain't no minor bug and should be fixed before the stable build launches.

This bug really should be fixed before the stable build launches (june 14th). I lost several 40mm grenades because there's no way you can stack them so they will disappear when you unload. @Geez Do we have your attention? :)

I've had the same issue but only when moving grenades I had obtained in the first EXP patch, grenades obtained in the latest patch seemed fine.

I can confirm this problem (bug?) exists but I believe it was already present since 1.17, maybe a little worse?
Makes hiding mines by placing clothes over it a real pain now :)

This bug still exists in EXP 1.16...

Update:

• It seems like allot of windows (if not all) in this building have this problem, also the ones in the tower.
• There seems to be no problem when you shoot from the outside into the building, so it's basically some kind of one-way mirror...

This problem/bug is still in EXP version 1.16. Also It doesn't seem to matter what gun/rifle you use.