I'm glad I came across this thread and happy (but not happy) to see others experiencing the same issue with stalling while riding.
I purchased my 2020 Striple RS back in March of last year and have put on over 6500 miles. Similar to many of you, my bike has stalled several times, mainly under harder braking conditions, downshifting with clutch lever from 3rd to 2nd. I would have to restart my bike while coasting. When I brought my bike in for the front brake recall, I mentioned to the Triumph service techs it had done it twice, randomly, and I was initially told it tends to happen with new bikes (I'm a first time Triumph owner but this not my first bike) and should stop after I put some more miles on it. I don't rev-match or blip my throttle every single time when downshifting but I have been more conscious of doing it, especially when downshifting from 3rd to 2nd. I found it has helped some and I went a few months without any issues, but last week, my bike stalled as I was coming off the freeway on the off-ramp and downshifting to come to a stop at the light.
Good to read about the ECU update. I will take my bike in to have it updated and hope that resolves the issue for mine. Looks like it has for some of you, so that sounds promising!
Thanks for everyone's input!