I am playing the current campaign without swarm because I have run into them too much
I have checked the generation a few times and it is fine. The swarm has a 1 in 30 chance of appearing when the required conditions are met.
1) No precursors or aether rifts generated in the system
2) At least one system body with a Swarm Mining Score of 4
The Swarm mining score for a system body is calculated as follows:
1) Total accessibility of minerals where the deposit is at least 1000 tons
2) Accessibility is counted as double if the deposit is 1m tons or more.
3) Accessibility is counted as 1.5x if the deposit is 100k to 1m tons.
3) Accessibility is counted as 1.25x if the deposit is 10k to 100k tons.