I initially assumed it was due to lag or something similar, but I decided to replicate it today and received the same result.
I went to ammunation and got ammo for my weapons two days ago (see video below). When I'm a civilian, I normally maintain 250 Deagle Ammo, 150 Sawn-Off Ammo, 3000 TEC-9 Ammo, 3000 AK-47 Ammo, and 80 (max 100) Sniper Ammo.
As a cop, I maintain 1000 Deagle Ammo, 1000 Sawn-Off Ammo, 13000 MAC-10 Ammo, 8000-9000 AK-47 Ammo, and 300-400 Sniper Ammo on hand.
When you switch from standard Civilian to Mechanic, Mechanic to Civilian, and Civilian to Cop, you get a non-cop weapon loadout.
(Video recorded 2 days ago and the same thing, had to re-buy ammo for all my weapons except MAC-10)
(Last job was
Mechanic (yesterday) and today went on duty as a Cop to replicate the bug)