I have the exact same problem: a TCL TV and a LG soundbar connected to "HDMI 4 (eARC)".
I've enabled "HDMI ARC" as outlined here: https://support.tcl.com/rokutv-setup-configuration/01-connecting-a-sound-bar-to-your-tv-using-arc
When I turn on the Roku, the LG soundbar switches to "HDMI IN" input, instead of "OPT/HDMI (ARC)".
Using the LG remote, we have to manually switch to ""OPT/HDMI (ARC)" EVERY TIME!
We've followed all of the instructions, but nothing helps - what a pain!
Come on Roku developers, you can do better.