I'm not sure if it's the altitude AGL or the way they spawn in, but I'll confirm that the manpads on this mission are way trickier to defeat than any other setting I've come across in DCS. Even with the proper break, flaring, engine to idle, I had to repeat this mission quite a few times from getting sniped by manpads. It's especially tricky since they just spawn in after you do the second attack tasking and are ready to fire. if you follow the given attack heading on that task, you will die 100% of the time. But on a replay you know it is coming, so you can break off and get some distance.
I don't know if this helps track down the problem, but I have a hunch it might be with the call out davy does of sam launch break. Without the mws even going off Davy will do the call out and then instant explosion. No MWS warning. I'm not sure if there is a scripting issue there, but thought I'd bring it up as it seems unique to this mission.
You can defeat them by just eventually memorizing where they all are and then sniping them with laser rockets from way far out, so it's not a big deal. Manpads should be pretty lethal to a low flying a10 that only has 2000 feet AGL to work with, so this mission is probably more realistic than others where a simple break and flare defeats them 90% of the time.
Really cool mission though!