Pretty simple, can't seem to go an hour without seeing it. Most common version is you kill the first 9 Headless ones and the 10th circle won't spawn, so you're just sitting there waiting for the boss. That said I've joined in progress and then had to wait 4 minutes with a total of 0 circles at all as well, and had it cut off around 6 or 7 on the odd occasion. Also an issue of once the increasing the spawn rate of circles it'll continue to only spawn 1. May seem minor since you can wait, but this is clearly old content that is simply being reused, the least it could do is work as intended especially when we have tonics that are based on time.