I've concluded my issue is as reported by two other users here
https://forum.openwrt.org/t/xiaomi-r4a-gigabit-21-02-problem-with-100mbps-ethernet-wan/107985
From my observations, the 100mb port negotiation issue with OpenWrt 21.02.2 does not affect ALL 100mb switches.
There is no issue when wired into gigabit ports of routers, and my Netgear FSM726 100mb switch has two 1Gb ports. I can't reproduce the issue using OEM firmware.
Update: problem with 100mb port autonegotiation appears to be fixed in snapshot. Early indication is my R4AG with 10 March 2022 snapshot when power cycled, is behaving correctly with the TL-SF1008D v9.1 100mb switch.