Same Problem on Pi4 with LE 9.1.502
LAN can not be changed to manual. It always returns to DHCP. I configure manual und after saving, it always returns to dhcp.
Under OSMC or Raspbian manual LAN configuration works without problems.
I'm a linux noob, but is it possible that this is a simple rights problem?
Edit:
When WLAN is disabled, i can configure LAN manual. And LE keeps settings, when i enable WLAN again and even after reboot.
But i can not let Gateway empty. It's like i configure ip and subnet mask manual and LE gets Gateway from dhcp or reminds a former setting. Strange.
The problem is, then there 2 Gateways. WLAN with internet connection, LAN without. So LE has somtimes access to internet and sometimes not.