I would like to +1 to @Jez Brook suggestion: I connect all devices through the AVR but once I connected an iPad via a USB-C to HDMI dongle to HDMI 3. I left the dongle/cable attached even though the iPad was no longer there — subsequently the TV was stuck and Simplink and ARC from TV apps were no longer working.
After resetting both TV and AVR CEC/ARC were still not working(!) - when I reattached the iPad and played some audio, everything came to life again and I could switch to TV apps. Come to think of it: it might have been "solved" if I removed the dongle as that seems to be some sort of smart/active device.
TL;DR - if you use ARC and have other devices connected via the TV (as HDMI-switcher) it might well get "stuck" at the last input/device. This is an OLED C1, BTW.