Why is the arc's shape so strange?

Even if you make modifications, the shape of the arc will continue to change like this when you reload the PCB file.
This can be solved by shifting the center point of the arc slightly away from the center, but why does this symptom appear?

It could be a bug. An arc should stay put where you put it, also after a restart of KiCad.

What KiCad version are you using?
Can you make a little test project and upload it here?
Simplest way is to make a copy of your project, and delete almost everything except the lines on Edge.Cuts.

Another option is to simplify it, and use a straight line instead of an arc. it looks like internal routing on a panel with mouse bites, and with a rectangular cutout, the router will just follow the inside of the contour as close as it can. Your final PCB will still have rounded inner corners with the same radius as the router bit used.

You can test with this uploaded project file.

KiCAD_Arc_Test.zip (60.8 KB)

This problem can be solved by slightly moving the center point of the arc.
But ultimately, this bug will have to be resolved.

Kicad uses simple arcs: sections of a circle, so you need to start the arc at a position that is at 90° to the end of the track.
Your start position is some distance from the end of the track, resulting in an angle somewhat larger than 90° so the radius followed will create a bulge.
Conversely, if you start the arc at less than 90°, the arc will not join at the end of the drawn line.

Apparently, arcs are treated differently in 7.99, but they are still simple arcs (sections of a circle).
What you want, with your illustration, is something more akin to bezier curves, which are not possible with Kicad.

Looks like a bug that’s been fixed: Imported DXF with arc malformed (#16089) · Issues · KiCad / KiCad Source Code / kicad · GitLab

I confirm it’s a bug.
When I select the arc, KiCad does show the correct end points as control points, but despite that it does not render the arc properly. At first I did not understand the control point with the question mark. A bit later I figured out it is the control point halfway the arc radius, but it does not show halfway through the arc, because the arc is not rendered properly. The control point itself is in the right location.

After that. I did:

  1. Preferences / Preferences / PCB Editor / Editing Options / Arc editing mode / Keep endpoints or direction of starting point (The other option is: Keep center, adjust radius) (This step was probably not needed).
  2. Edit the arc, set the Arc Angle to 180 degrees.
  3. Save the PCB.
  4. Exit PCB editor.
  5. Start PCB editor again.

And then I have the faulty arc’s back.

A quick workaround is to set the arc angle to 175 degrees. Apparently it’s something about the 180 degrees that triggers the bug. (179 degrees may work to, untested).

Just recently, davidrsb mentioned in this thread: Circle once more - #6 by davidsrsb

So regardless of whether it’s a bug in KiCad, it’s probably still best to avoid 180 degree arcs in your panel.

What triggered the bug was the mid point Y coordinate being equal to start/end point Y coordinate

I have been experimenting a bit with your panel, and added a few remarks.
I’m not a paneling expert, but I guess you will agree it makes sense when you read it.

  1. More width for panel strength.
    Maybe 20mm or so?
  2. Smaller panel through nesting.
  3. Remove mouse bites. Set arm free.
  4. Diagonal break line for panel.
    Add routing to balance ease of braking
    with the panel strength.
  5. Remove all mouse bites on the arms.
    The arms are fragile, you do not want to risk breaking them
    during de-panelization.
  6. Better: Move the mouse bites closer to the center of the body.
  7. Add few mouse bites.
    Not for strength.
    Only to keep the center square from falling off.
  8. Do not make all mouse bites equal in number.
    Balance strength versus ease of separation for each joint.
  9. Strong mouse bites for extra panel strength.
  10. Break panel over blue lines.

And the modified project with the notes:
2023-12-14_KiCAD_Arc_Test.zip (60.1 KB)

Another small modification. I have eliminated the waste piece in the center by shifting the left side a bit the Y direction is even smaller, but the X direction is a bit wider. Consider panel integrity during SMT placement. The panel is usually supported on two ends, and making this dimension shorter increases stiffness.

2023-12-14T10:11_KiCAD_Arc_Test.zip (60.2 KB)

When designing for bigger (mass) production runs, the size of the base material is also an important consideration for your panel. Take as example a (fictive) panel size of 600x600mm. If your own panel would be 200x200mm, then you could get only 4 customer panels out of it (because of the routing with that subtracts). If your own panel was 198x198mm, then you could get 9 customer panels out of it. That is more then double the PCB’s for the same cost.

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