Did it 'bounce you out' at a 6pm or midnight restart, or did you just disconnect or fall off the server due to network issues of your own, or someone hacking the server and failing the mission?
Also, how shortly before you got bounced, had you purchased the pistol? And had you done a manual 'sync data' from your Y-menu more than a minute or so before you got bounced out?
Unfortunately, if you fall off the server (or get booted or restarted) before your item configuration has been banked to the database server, then your recent changes may be lost - this is unfortunately the same for all Altis-Life servers as item config storage is not instant. On our server it happens at ten-minutely intervals automatically, and can be manually synced using the Y-menu, but even so, times can happen when things disappear, if players miss the restart warnings or fail to sync manually.
In very rare cases - sometimes if you are carrying or wearing other items that your player-class is not legally entitled to possess (such as cop-gear or donator-gear) - it is possible that either of the automatic or manual syncs may fail (silently, annoyingly), and your config not be completely saved. This is actually on my list of things to investigate, as it happens - I only discovered last night that the core Altis Life system may have a blackhole there in those cases. However, it's unlikely to apply to you in this instance - most likely you got the pistol shortly before a restart or fall-off, and it simply didn't get saved manually or by the 10-minute cycle. I'll put in a good word to the Admins - I'm sure they'll sort you out for a replacement weapon if you continue to ask nicely!