This seems unreasonable...
I think there was also some associated bug. Blackrock was eliminated from the sector but had a resurgence. Immediately after the resurgence was successful, this raid was sent. I then went and captured the only Blackrock world, eliminating them again, but the raid was not canceled even though it was still 70+ days before the raid gathered when I captured the planet.
Hm. Well that is a lot of fleets, but I expect it wouldn't spawn that many if the defenses didn't have a chance. How many stations & military bases do you have in that system, and do they have alpha cores?
On the raid not being cancelled: It's coming from the resurgence base, not the planet... which it shouldn't do. Fixed (not retroactively, sorry!) in above hotfix.
I have 2 max stations and 2 high commands in the system and I think I put some alpha cores in them...
But wait, if the attacking fleets just scale to whatever defenses you have, then there's no reason to increase your defenses... I thought putting alpha cores in them would make it so I wouldn't need to babysit, but I guess instead it means I have to chain battle 18 invasion fleets... fun
Maybe the number of fleets should be capped or something.
edit: it also seems like the maximum strength of invasion/raid a faction can send should be related to the strength of the faction. Maybe number of markets or total population at markets with heavy industry or something. It just doesn't make sense that single world (where every industry is disrupted for 100+ days because they just overthrew the government) could assemble an 18 fleet death ball.