The internals of an add/remove set CRDT is monotonic, because we only ever add to the internal sets, so the internal state of the CRDT cannot ever go backwards in logical time. 

However, the observed state of the CRDT is that we're adding and removing elements, so the observed state doesn't have to be monotonic. 

If we chain these systems together and take actions based on the presense or non-presense of an element, it doesn't look very monotonic anymore. The final state will still converge eventually, but we may or may not see some elements for a while before it stabilizes. It's not unlikely that some side-effect happens because of that intermediate state, such as a user reading the state of the system and reacting before it converges. 

What does it mean for a CRDT to be monotonic?