Even though patrols spawn "rarely" now, they will spawn very strangely in certain cases. In the day they spawn very often since monsters are forced to spawn only in caves while the illagers have few spawning restrictions, boosting their spawn rate. On superflat during the day, over a hundred illagers will spawn (until the hostile mob cap), drastically decreasing slime spawns to only a few slimes. This can create a huge illager army with multiple leaders and sometimes this army can even be created in a default world. As far as I know, a patrol isn't supposed to be a giantic army!
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Relates to MC-139048

The illager patrols spawn too much on a normal world as well, in my opinion. In the latest snapshot I keep getting swamped by them at my base and I am also no where near a village. The closest is over 200 blocks. Its very annoying. I wish they would spawn near villages and only two groups at any one time.
I have heard that one of the developers plans on changing the spawning of illagers in the next snapshot (which should happen later today). When this snapshot comes out, I will observe the new spawning mechanics and update this ticket accordingly.
Apparently they no longer use the same spawn cap as monsters. In fact, they don't spawn at all (in superflat). See MC-139817

Is this still an issue in 18w50a?

This issue should no longer be valid due to how spawns are done now for Patrols
Agreed that this is no longer a problem due to the more balanced spawning of patrols.