Great, many thanks for the useful feedback. Good to know these details!
I think that the interactions of the different FW’s (Tasmota on the Bridge), factory FW on the S31 ZB Lite and HA have caused these issues. By introducing the IKEA TRADFRI in the mix, it very likely helped indeed the S31 to be able to pair to the ZB Bridge.
Btw: Sonoff is very much focussed to miniaturize their devices as much as possible. Probably to make those more appealing for the larger public and perhaps also to minimise shipping costs (most of it they ship outside China).
With that, I believe they compromise battery capacity of the sensors and the effectiveness of the antenna printed on the board.
Did you also open up the IKEA TRADFRI (is it easy to do?)?
If so, could you post then a few pics of it?
Would be very interesting to see the internals and how the antenna design on the print board looks like.
Cheers