Strange Breaker Behavior with Treadmill Electronics
The electronics of the treadmill are putting out &quot;noise&quot; that the CAFCI breaker is interpreting as a potentially dangerous arc. This can be caused by all kinds of things, including motors, but in this case the motor is fine and it is &quot;no motor&quot; when you have a problem. My hunch is that the noise of the electronics gets &quot;lost&quot; when the full load of the motor is in use but when it is by itself it stands out and the CAFCI treats it as a problem.
A few possible solutions:

Check with the treadmill manufacturer. They may have a fix.
Put the treadmill on a heavy-duty surge protector. Make sure it is rated to handle the full load of the treadmill. A surge protector is primarily to protect problems in the incoming power from affecting a device, but it can help with problems in the other direction too.
Check with Siemens. They may have newer/better CAFCI breakers that you can swap. A newer design may handle the noise more selectively, preventing false positives.
Check other breaker suppliers. Carefully. You can't simply &quot;swap breakers from different brands&quot;, but there are some situations where one brand will make a breaker that is listed/certified for use with another brand. In other words, it has to be specifically designed to be compatible with your Siemens panel.