Built the P+N version on one station to test: battery current in sleep dropped by about 0.1 mA, and the reading is the same as before within 0.02 V. Changed the rule to the one above. Two sunny mornings now, no flapping. I’d had the e-mail recipient on a personal address, moved it to the team mailbox while I was at it.
Modbus is just registers until it isn't.