This bug is still not fixed properly. Sometimes it works like it should and sometimes it does not. In 1.19 (latest EXP patch as for this moment) I've managed to ARM it the second try on two different occasions.

And again: Same server, a different base but again a gate in a doorway:

So, I've found another base on the same server in a different building and it happened again:

No, it's a vanilla community server (Dusk) that doesn't require you to load any mods. It only has a few small tweaks like a little bit more stamina and some old (pre 0.60) SKS/Mosin skins but those should already be in the original Dayz files.
Maybe this bug has something to do with the gate slightly clipping through the metal door behind it which you can't see. My guess is that the game thinks the gate is behind the metal door but visually it's (obviously) not.

I've made a video about it:

In T166066#2333169, @sileed wrote:

we also had players report that the same shed always spawns multiple plastic explosives. Wiped our test server and sure enough that same shed is still spawning multiple explosives. We do not have loot turned up (in fact some of it is turned way down) but it seems the world doesn't have enough space to place these industrial tagged items so it constantly chooses the same places.

I don't find THAT many, I think it's fine like it is at the moment.
Also remember that we need to be able to find enough of these explosives in order to be able to raid/destroy bases as grenade damage to doors/walls has been nerved by A LOT. Bases cause huge server performances issues and therefor the amount of bases needs to be reduced to a minimum.

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.