I know it’s supposed to be random and more or less based on the kingdom at times but next to every explore i’ve done (Xbox 1) always have a fortress gate - gob chomper even sheggra and not even in Broken Spire for exploring i’m pretty sure this shouldn’t be happening? Maybe lol
By Event troops I mean the buffed one’s (as of this week it’s Broken Spire troops and Giant’s)
Short version.
Explore uses troops list from the Kingdoms respective Challenges 75-80%ish and Quest line 20-25%ish.
ONE troop is “randomly” replaced. With a weighted bias given toward the Weekly Event.
This week you’ll see more Broken Spire.
But it’s every single time? And every single kingdoms explore? On PC 0 Suncrest troops in any kingdoms explore 
I know how you’re feeling. I was grinding some explore this week (I probably did 150 explores in Stormheim and Mist of Scales) and it felt like the only 2 “random” troops that were coming in were Stone Giant and Fortress Gate. I know I was seeing Ettin, Ogre and Rhynax, too, but not once did I see Terraxis, Luther, Fire Lizard, Obsidian Golem, Lava Elemental or Sheggra. A handful of times I saw Gog & Gud, and Gobchomper, but they were very rare.
I used to see the occasional base-mythic troop (Horsemen), but I don’t think I saw any this week.