Mobs getting spawned over and over. Friendly as well as hostile.
Update by @unknown:
It seems plausible that the mob cap is being ignored.
I tested an area with mob spawners and they continued producing new mobs even though there were many in the immediate vicinity.
According to the wiki:
If, at the time of spawning, 6 or more mobs of the spawner's type are present within a 9×9×9 area, centered on the spawner block, the spawner "poofs" without creating any mobs and then waits for the next cycle.
Screenshot:
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This is a bulk resolve of issues that haven't been updated to 0.16.0+ yet. Please comment below (To open the ticket) and update your affected versions. If you cannot reproduce the bug, please write in the comments so we can resolve it as "Cannot Reproduce". Thank You!