After a prompt response from Seeed support and a few days of trails it appears that it is the transmitting power that is the issue. When the transmitting power is set too high the connection becomes unstable.
After testing, a setting of WIFI_POWER_17dBm or WIFI_POWER_15dBm seems to work well for me. Somewhere it was recommended to put the setting after the WiFi.begin() statement (not tested if this is needed)
WiFi.begin(said, pwd);
WiFi.setTxPower(WIFI_POWER_17dBm);