Pyrex

For some reason Cura is placing supports under the first layer of a model that sits flat on the print bed. Why is this happening?

For some reason I've run into this problem with cura on multiple occasions. Cura is placing support under the first layer of my model, it makes no sense. This bug has happened to me a few times before and I could overcome it with adjusting the support angle, but on my most recent model I can't. Can anyone explain this?