Sigh ... I found the source of my problem and it was one of my own making and not a problem with rollback. But, I will share the details here in hopes that someone else my encounter a similar problem and that this may help them to find a mistake.
I had thought that I was making a change to the Start and End of a calendar event, checking to make sure that change was legal (in this case, checking to see that it did not overlap other events) and then either rolling back if it did overlap or commit if it did not.
What I failed to realize was that I was also calculating and storing the new duration of the event (the difference between End and Start expressed in hours). In that Change Object I had accidentally selected the Commit "Yes" rather than the "No" option. As a result, when I later decided that I had an overlap for the new calendar Start and End times, rather than rolling back to the original Start and End times .... which is what I wanted to do ... I was only rolling back the calculated duration amount.
In short, the Rollback was working properly, but that extra "Commit" during the Change Object caused me to roll back only to that last commit rather than to the original Start and End timestamps my entity possessed when I started the Microflow.
So, my bad, mea culpa, stupid me ...
At the very least, I hope that someone else will see this at some point and either avoid creating this problem or at least be able to more quickly identify it.