The bug
Pillagers riding ravager, spawned in a raid, will take up the pillager mob cap. Because of this, captains won't be able to spawn inside outpost anymore. Normal pillagers spawned in a raid do not have this behavior.
Captains can only spawn in outposts if no pillagers are inside the mobcap. The pillagers riding the ravager takes up the mobcap, while normal pillageres spawned during a raid do not.

Steps to reproduce
1. Open Pillager on ravager test.mcworld
2. Press the structure block that spawns a pillager riding a ravager
3. Watch through the glass how captains are not able to spawn
4. Kill the pillager riding the ravager and see how captains are able to spawn again
Note: You can use the command blocks to kill the pillagers

Extra steps
You can also use the second structure block to summon the pillager that spawned inside a raid. You can watch how a captains are able to spawn while the pillager is alive.

Media
In the video that is on YouTube (because of size limit on the bugtracker) you can watch the steps being reproduced. Watching this video will also help you to reproduce it yourself

YouTube link: Instruction video about pillager on ravager taking up the pillager mobcap