When Any-Planet-Start is installed and the starting planet is set to a planet in planet_stuffmapping (e.g. Fulgora), no imersite asteroids get added to that planet's orbit. In data-updates.lua, the elseif branch that handles the APS starting planet has the table.insert commented out on line 47. The condition correctly filters out medium asteroids, but the insert that would add chunks and small asteroids never runs:

elseif (string.find(def.asteroid, "medium") == nil) and (planet.asteroid_spawn_definitions ~= nil) then
--table.insert(planet.asteroid_spawn_definitions, def)

Uncommenting that line fixes it. :)

Sorry, this seems to have fallen through the cracks. I believe, I tried following your advice and uncommenting that line, but it ended up breaking something else.

I might return to it later, but in the mean time, it seems that "start on Fulgora" mode is just happens to have a higher difficulty setting. One needs to actually unlock Aquilo first to harvest the asteroits en-route to it, as the mod originally intended.