If playing on Easy with the default “Item” settings, guns, magazines and ammo crates spawn with random amount of bullets like on Normal difficulty.
This definitely was a thing where guns and mags always spawned full on Easy.
I think when Nelson added durability options for different item types he also made guns and mags that spawn on Easy to have random amount of bullets with only a chance for them to be full like on Normal
This is controlled by the Magazine_Bullets_Full_Chance difficulty settings. It’s been a 10% chance on Easy (5% for Normal, 2.5% on Hard) for quite a while – at least since 2016. I’m not sure this ever had a 100% chance for Easy.
You can definitely customize your difficulty settings to make it guaranteed though.