I have yet to come across a good solution to this issue, but I have created a workaround in my flow (which I don't like because it is very inefficient, but it is all that I've been able to come up with yet). The problem is that after an Outlook event series is created, any modification to individual events in the series (whether it's updating a single event or deleting a single event) is treated the same by Outlook. When you do this Outlook doesn't send out a single trigger for that event but rather sends out individual triggers for every event in the series as an update trigger, where every event that you did not update just gets "updated" with the same info it had before. It's a really inefficient way for Microsoft to handle this with Outlook events that they really need to change.
In the meantime, my workaround is that I added a column to my Sharepoint calendar list to save the data from the trigger header called "xMsRequestId" and now every time a new event is created or updated I have that Id updated for the event as well on my Sharepoint list. This is a unique identifier for the action that caused the trigger, and when all of those update triggers happen for all of the events in the series they all have that same Id in the header.
So here's what I use it for: whenever a trigger for the flow is an event update, I check if it is a part of a series (by checking if the trigger output "body/Recurrence" is not "none"). If it is an update to an event and if the event is part of a series, then I do a 'Get Sharepoint Items' for my list filtered on both the 'body/Series master ID' and and that are not equal to 'header/xMsRequestId'. This indicates that those are stale events since their last update was from an earlier xMsRequestId. I then tell it to delete all of those stale events. This way as all of the additional triggers flood in for all of the individual event updates, I then see that those don't exist yet and add them as new events in the list with the new xMsRequestId. At the end of all of the triggers completing their way through the flow, you are left with all of the events in the series being refreshed on my Sharepoint calendar and the one that had been deleted in Outlook is now missing on Sharepoint as well since it did not have its own update trigger.