Yes, I did try changing IP address on PC to be on new subnet to see if I could see the hub on new LAN IP address, but it made no difference, as hub was not visible on new IP address.
As far as I'm aware, I think the 'Apply Unchecked' dialogue box should appear 'before' the LAN IP address is forcibly changed on the hub, when it has worked.
No luck yet in finding a way to disable auto-roll back after searching github. Looks like it cannot be disabled, but I could be wrong.
Update (2 Oct 2018):
Turns out this issue happens on HH5A when WAN and LAN are configured with same subnet.
eg.
LAN 192.168.1.1 (default)
WAN 192.168.1.209 static IP
LAN IP address cannot be subsequently changed using LuCI under such conditions.