Steps:
Expected behavior
That's currently the intended behavior for handling synchronization conflicts with documents. Merging turns out to be very complicated to handle correctly.
Nice bug I was able to repro this. Thanks.
Edit: Okay nevermind.
For documents, that totally makes sense (although not the best).
For reading plans where no edits to the plan's parameters have occurred, only more reading completed, it makes no sense. Whichever computer has read the furthest is the one whose plan should be used.
Even if this would be a tough bug to tackle before release, the behavior related to reading lists is a bug, as it is