Hooray for your techno-sense!!
I would not be surprised to find your thermostats had / have a low battery icon on the display. Which isn’t very obvious unless one is in the habit of scrutinizing one’s thermostat regularly.
The last set of good Honeywells I had had a bright red light that turned on for low battery, loss of program, or filter needs changing. You might not notice that red light instantly, but within a day or two for sure.