They are spawning. But 1st of all rarely and 2nd not on every military on the map.
I recently found a few mines in Tisy

Onemiller11 yes, I'm aware of the tier system. Tisy and the military gas station are the only places that can spawn landmines on vanilla Chernarus, other than the random tier 4 military crates in nearby towns. Yet I have looted Tisy up to 8 times on my main official server at one point and didn't find any within that 4-5 month span. That can only mean that the landmines that are set elsewhere near someones flagpole are keeping them from spawning. If you know what landmines are best used for you could start to understand that any item in the game (once interacted with) should not effect how many of that item spawns after that point.

Even with vehicles there should be no exceptions, vehicles should spawn slightly less to compensate. The ammo case should be extremely rare but again no capacity. This will make it to where you don't go weeks without seeing one car. I didn't even know ammo cases where a thing until a year after I first got the game. That goes to show how the loot is effected..

I found one yesterday, so yes they are spawning.