I have an MT1300 Beryl - I just flashed it back from generic OpenWRT - and installed 4.3.6.
When I went back to the LUCI Console - I was trying to modify TTL in the custom rules area of the firewall.
I noticed that the tab is missing for custom rules. Has it been placed elsewhere or did I miss something? I rolled back to 3.216 and verified its back.
FWIW I’m in the habit of using the GL GUI & LuCI only when using Private Mode/Incognito as I’ve found web browser caching to be a strange source of annoyance/apparent bugs like you’re experiencing.
UCI is also stock in the GL firmware if that helps.