As most users were pointing out on the comments of thing:2912690, the circle part was separated from it's attaching bit on the bottom.

Now it's fixed and you can print the whole thing with a little less frustration.

This is just the circle part of https://www.thingiverse.com/thing:2912690
If you wanna print the whole thing, download everything from there and just download the circle from here.
The author is free to include this fixed part in his thing, if he choses to.