ESPHome changes how ESP8266 boards receive software-driven serial data so the incoming bytes do not hold up Wi-Fi long enough to restart the device. The correction applies at 19,200 baud and below; faster connections keep the existing method.
J. Nick Koston reproduced the problem on a D1 mini receiving serial data while repeatedly reconnecting an encrypted client. At 19,200 baud, the old version restarted three times in four minutes. The revised version ran the same setup without a reset at 9,600 and 19,200 baud, although some byte errors remained during reconnect bursts.