@gavin.landon
Hi, and welcome to the group!
I'm very new to Fusion mysef, but a lot of the behavior is similar to Inventor which I know very well. Your circle needs to be constrained to the horizontal origin line. You visually placed it there and the snap made it appear to be locked there... but without an actual constraint, it can float off of that line. You would need to prject that horizontal plane, and then either constrain or dimension to it the same as you did to the vertical origin.
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Chris Benner
Industry Community Manager – Design & Manufacturing