The RNG in this game is so strange.
For example. I fly up to a planet with an artifact discovered, land, steal it, defeat light resistance, and get item X. I reload, repeat the process several times, never getting item X again, only Y, regardless of if the mission was accomplished full stealth or not. As a result of savescumming the Artifact thefts, I have 3 Plasma Guns that glow with an eerie blue light, as it kept being the "second" result I'd get after loading a save. The other artifacts I found that aren't those specific Plasma Guns were on non-Mechanicus planets (and didn't save scum to change the results.)
I fly to a planet with an undiscovred POI. It's an artifact. I dislike the outcome of the battle I fought to secure the artifact. I reload. The same planet now has an STC fragment, and no amount of reloading changes that.
I play 50 turns with perhaps 1 "major" (ie. has a screen of its own) event the whole time. (A WAAAAGGGHH, for example. But also stuff like Heretic Fleets.) Then, after some mystical time (coinciding roughly with the death of all the Ork hordes in the sector) big events start popping regularly. I have a save on the crux of one of these events, and within 2 to 3 jumps it happens in the same place every time. It starts out with a smattering of events (Gene Seed Mutation, The Maw Of The Warp Opens, Crusade Events, Inquisitor Missions, Promotions), and as I keep reloading, the results get narrower and narrower, until finally no event but Gene Seed Mutation pops.
Battle results seem pretty repeatable too between reloads, although there's more going on there surely. Still, playing Blood Angels, I've reloaded battles and had the same same # of casualties and Red Thirsts and even Red Thirsts affecting the same marines as in previous reloads.
It kinda makes me think the seed is on an interval of some sort, and that there's something going on with seed generation when you reload a game. Like it pushes the seed forward an increment and it gets saved into your game, so you can't get the previous result you just had (usually.) This is all purely anecdotal navel gazing and I'm probably wrong on all of it, but as someone who likes to reload saves and explore all possible alternatives, I've found myself abnormally frustrated with this game. It tantalizes you with one result, but then sticks you with another repeating result the second and nth times around. I think I spent an hour reloading the same small moment in time (measured by a ship carrying my whole Chapter), because an event kept popping with such regularity. It got to the point I reloaded an earlier, played it up to the same place and started again, getting a completely different (yet still repeatable) set of results from it.