Not Roku's fault.
Some AV Receivers/Soundbars play nice with Roku TVs. Others do not.
I have two '17 4K Roku TVs - same brand.
One ARC HDMI works great via Vizio soundbar.
But the other is hooked up to a Yamaha AV receiver. It errors out whenever inputs are switched on the Roku TV and it attempts to decode the new stream.
Good news is a workaround exists:
Disable ARC within AV Receiver's settings, but keep CEC/System Control on (that way the volume buttons on Roku TV remote continue to control volume and power).
Keep HDMI ARC plugged in for both equipment.
Also use a SPDIF Toslink cable and set receiver to the SPDIF input.