OK - hold the boat.
I finally figured out what is going on. I was on the right track when I concluded that something had to be done on the BM side.
When you change from duplex=0 to duplex=1, BM automatically changes your hot spot configuration to have two time slots.
You will not see this unless you are looking for it - e.g., by going to BM self-care and refreshing the browser (may have to clear cache in some browsers).
Any static TG you had configured will "disappear" - it belonged to your old duplex=0 hot spot.
You need to set the static TG for the appropriate TS for your shiny new duplex hot spot. Otherwise MMDVM_Bridge will be disconnected from receiving the TG after a few minutes.
>> this was what was causing the issue I observed.
Thanks for humoring me -