I also encountered this bug the other day, and found a workaround:
Add &order_by=login to the end of the query.
This will order by user member login names rather than by observation count, which is the default order, and seems to solve the problem of missing users.
I found this by looking at the code here.
My guess at the reason behind the behaviour is where one page ends and another starts, if there’s users with the same number of observations, it’s possible to receive the same user at the start of one page, and the end of another, which effectively hides the user that should have been returned instead.