I often sync Octo to ext midi clock, but I guess I have not done it using clusters. Because this is what I found today:
Page mode, ext sync on, two adjacent pages in a row, cluster play off. The Octo starts in a predictable way every time.
Now turn on cluster play. When started (as before externally) the first page briefly lights and then it moves to the following page, creating an "out-of-syncness" that even varies a bit with a slight random amount.
I am afraid that this renders clustering in ext sync pretty useless for now, so I would suggest to give this a rather high priority.