There’s two different things going on here. The first one, where you can’t split out the walking segment from the car item, is due to the processing engine’s rules on what qualifies as a “keeper” timeline item. Those being: at least 60 seconds duration, and at least 20 metres distance.
So when you try to extract that walking out as a separate timeline item, Arc will go ahead and do as requested, but the processing engine will then kick in and say “ah but that new item doesn’t meet the requirements, so I’m going to merge it back in to the adjacent item”. Quite annoying!
Most of the time that rule is actually beneficial, because it ensures the main timeline doesn’t become cluttered with inconsequential details. But ideally it would be great if there were a way to override it, telling Arc “actually I for reals want this segment to be its own timeline item. plz don’t overrule me”. That’s something I’d like to add as a feature in some way, some day.
The second thing going on is that stationary segment that you can’t get to attach to the adjacent Visit item. What’s happening is that stationary segment is made up of samples that fall too far outside the radius of the visit. So when you extract them out and assign them to the correct Place, the processing engine then ejects them from the visit again, on the basis of them being “outside” of the visit.
A common situation you’ll see this happening with is with buildings where you might dwell briefly in an entryway or antechamber, like in a driveway or foyer or lobby or whatever. That brief bit of lingering at the start or end of the visit, on the peripheral of the building, will create a bunch of samples that cluster outside of the main visit, and the processing engine will keep refusing to merge them in, on the basis that they fall outside of the visit’s gravity well - acceptable as a satellite but never permitted to land.
You can work around this by assigning the segment to a different Place. For example for me I’ve added private places for things like “Bicycle Parking”, for where I park by bike outside of my building, or “Hotel Reception”, etc. That way you can make sure that stationary segment doesn’t pollute the trip item’s duration/speed/etc, and also add extra useful detail to the timeline.
There’s also an unfortunate extra case though, of when the clustered samples at start or end of the visit really weren’t on the peripheral of the building, but quirks and noise in the location data have made them appear that way. Sometimes location data isn’t great. For those cases unfortunately there’s no useful workaround in Arc yet.