Look around the starmap for Rebel/chaos legion, doesn't matter what the task is, that specific map holds 2 side quests which are rescue some guardsmen and watch a psyker in front of a portal.
I did a few maps with the key hole map piece, that was in the bottom right corner.
Gave me both guardsmen and psyker side quests.
Got some of the needed maps on hive planets, agri-world and industrial worlds.
If it can help other peoples.
I'm still at 8/25 to unlock suppression tree.
Done several maps with the 'key' zone : no event, no guards.
I guess side mission are bugged as hell, or that the mission generator restrict them at a crazy rate.
And still no words from Neocore about this ...
There is also another thread about this, with an actual screenshot of that map, but you'll need to search for that yourself.
which looks a bit like that one, slightly bigger, no crossing in the middle.
There is another with the "key-hole-map-piece" at the upper right corner that holds 1 side mission (guardsmen).
One thing they all have in common, is that they are all "tropospheric" maps, which can only spawn at a handful of planets.
Look through the star map, identify some of those planets, and if you are determined to do it that way, accept and then surrender mission that you don't need, to make those tropospherics missions spawn.If one planets runs dry of missions, go to another planet.
It's still not super fast to do, but faster than running the campaign a total of 5 times to get enough side missions.