The spinner is the bug
Tapping a checkbox and watching a spinner teaches people that the app might not have heard them. They tap again. Now you have a double-write problem and a user who does not believe the list. Writing locally and syncing behind the scenes removes the whole category.
Conflicts are rarer than people think
Two people editing the same field of the same task within the same minute is genuinely uncommon in a team of ten. Designing for it as the normal case makes the common case worse. We take the later write per field and keep the earlier one in the task's history, where it can be recovered.
What the user actually sees
Nothing, most of the time. That is the goal. When a device has been offline long enough to matter, the app says so plainly at the top of the list rather than pretending everything is current.
The cost
Every feature has to be expressible as a local mutation plus a reconciliation rule, which rules out some server-clever designs. We think that trade is worth it for a tool people open twenty times a day.