The workout starts from home and ends in another location (I walk to another place). After adding it to the timeline 5 days ago, my home circle became significantly larger than before. I removed it from the timeline, but the circle is still large (place model update completed 2 days ago).
I reviewed the visits to my home location and can confirm there is not any incorrect visit assigned to it.
It seems that workout data messed up the timeline and made Arc consider workout as me at home.
Place centre and radius is only calculated from samples inside visits, so the workout’s timeline item won’t be able to contribute to that calculation. But clearly something’s gone wrong, so we’ll need to figure out what!
Place details aren’t updated immediately, they’re scheduled as a housekeeping task that gets run later, possibly overnight (or whenever iOS decides to run the scheduled tasks). So you won’t see your actions immediately reflected in the place’s centre and radius, you’ll need to wait up to a day at most (though Arc does ask for the task to be run within I think the next hour, but iOS doesn’t have to respect that request, and typically ignores it).
My guess is that some other visit temporarily got incorrectly assigned to your home plan, which you’ve since corrected, but the timing was such that it made it appear as though the workout item was involved.
Anyway, I think the best next step will be to arrange your data the way you wanted it, with the workout in the timeline, then wait a day to see if the place data updates correctly or not. If not… we keep digging!