Welcome from Cleveland, Ohio!
My 5 week old 2016 Roadster stalled TWICE, exactly the same way as yours did. Both times I was downshifting to come to a stop and the engine just died. The electrical system was still active, the engine just quit. The weird thing is that both times this happened it was at the same intersection at the same approximate time of day! The bike started right back up without any more problems.
Last week I took it in for the 500 mile service and mentioned this to the dealer so he could check for problems. Nothing was found, no failure codes were noted. It hasn't happened since so I'm keeping my fingers crossed.
The 50/50 87/93 octane gas mixing is also interesting. My dealer told me to run at least 89 octane in the Rocket so that is what I have been doing. Yesterday I decided to try the 50/50 87/93 mix to see if the bike would run any better....now I find out that the group says to run plain 87 octane. Why is running higher octane gas bad for the Rocket?