I'm not sure if this is something you already know about or not. There seems to be a conflict between some of the custom weapons and vanilla weapons, pistols to be specific.

Last night I was on the server playing as a Triage carrying the XD40 on the Silent Hill map by Swift-Brutal-Death. Leaving the trader I saw a spawned Handcannon on the ground so I picked it up with the intention of selling it after the current wave since it was too late to return to the trader, I tried to draw my XD40 once I reached our camping spot and pulled out dual handcannons instead, I checked my inventory manually and found the XD40 had been replaced with the second handcannon and was gone.

I'm not sure if carrying a P280 would produce the same glitch, but it seems likely.