I understand exactly the scenario as you described. Dev has identified this problem a while back that the date range does not always correctly reflect the actual Start and End Date of an event in the month view mode. This was not always reproduceable and happens probably 50% of times when the page is refreshed. Switching to another view would sometimes fix this, but not always.
We did introduce a fix to address this rendering issue found in the calendarview in v5.5 and that is why you cannot reproduce this behavior in 5.5.