@William_Ishiwata There are a few issues that can affect the roundness of circles. As you mentioned, the file itself can be the issue, or more specifically, the software used to create the circular object and how that software renders it. Some apps do better than others. There are also many settings within apps that could also affect the "roundness" depending on how they are adjusted. Remember that our 3D models are really just a series of polygons stuck together. If that polygon count is rendered too low, then you end up with those octagonal looking cylinders rather than circular.
Mechanical factors can also come into play though, so it cannot be completely limited to software. A partial nozzle blockage or filament feed issue can cause a kind of intermittent edge, which may look like a less than round circle. It could also be a problem with the LM bearings or smooth rods being misaligned and causing carriage shift. On delta printers like the FLUX, a problem with the effector rods or joints could theoretically create angular looking circles too.
There is no easy answer, so the best way to diagnose is to check files or create a few from different sources. Start with a freshly cleaned and lubricated machine to try and eliminate mechanical issues as well, and go from there.
@Tiwaz Could be bearings or could be ringing. A bearing issue may not be evident right at the start. Remember the FLUX uses the long version LM8LUU so the first cm or two is barely hitting its stride and the bearing hasn't had to move much. Printing too fast can also cause that same kind of result though, so that is also a possibility. Or if the printer is not on a stable base and a wobble or vibration gets going, it will transfer right into the print. A lot of variables, process of elimination is the best way to figure out what is going on.
When printing, do the carriages look like that are twisting or moving out of level? They should stay perpendicular to the base. Are you seeing a lot of your lubricant turning black on the linear rods or on the ball ends?
What speed did you print that part at? Does the same pattern occur if you print slower? Is it always the same, i.e. reproducible at any speed?
Is your machine free-standing? Can you stabilize at least one side against a wall? Is it on a solid table?
Let's try to narrow down the possible causes.