ITead’s “Sonoff Zigbee 3.0 USB Dongle Plus V2” (model "ZBDongle-E") based on Silicon Labs EFR32MG21 radio SoC/MCU

That was a wrong presumtion; no versions of the Sonoff ZBDongle-E from ITead support. It only works because firmware will fall back to using no flow control if the hardware does not feature support for it.

Recommend flash ZBDongle-E with firmware that has software flow control (better than no flow control).

PS: The Sonoff ZBDongle-P from ITead does support hardware flow control but you need to flick the DIP switch on the board to enable it → ITead's "Sonoff Zigbee 3.0 USB Dongle Plus" (model "ZBDongle-P") based on Texas Instruments CC2652P +20dBm radio SoC/MCU