Most likely intended and most likely a duplicate of MC-167377.

Illagers with banners, commonly known as "raid captains" or "patrol captains/leaders" can be spawned by a spawn egg of an illager type. It also works with the /summon, making the unused illusioners spawn as a captain/leader as well.

To reproduce

1. Spawn a pillager using its spawn egg or use the summon command
   → Some of them may spawn with a banner

Only vindicators, evokers and pillagers and illusioners are affected.