I can’t say specifically what’s causing the issue, but I have a few ideas maybe.
How large is the map persay? Draw distance plays a large role into how particles/how many particles get drawn. Farther distances have a tendency to remove a majority amount of particles, even at full graphics.
Are the particles emitting from points? if that’s the case, especially for larger particles, if the point is obscured the particles might be completely negated from the screen.
And on that note, how large are the particles? big big ones generally have rendering quirks.
From what the scene is looking like, you have pretty large and abundant particles (I assume the big black cloud is one) I would tone it down or increase opacity to lessen the amount, alternatively, you can use billboard UI’s for virtually the same affect in most cases.
Hope this might help! I can’t really say a specific solution as you can go about it in a bunch of ways.