RPi3 - Using LG Magic Remote through Denon Receiver

Hi, I’ve just got a new LG TV (SK8500PLA) to replace a Samsung.
The HDMI Arc channel connects to a Denon AVRX2500H.
When the Samsung TV was connected and the channel on the receiver set to OSMC input for the Pi, I could control the TV with the Samsung remote. But the LG remote doesn’t work.
But if I take another HDMI input from the TV and plug that into the Pi, I can control OSMC fine from the LG remote.
Does anyone have any ideas what might be the issue and if there is a way I can have the Pi connected to the receiver and control with the TV remote, or do I just need to use another HDMI input on the TV?