I have a TCL 55R615 ROKU TV connected via HDMI ARC to a Yamaha RX-V377. On OS 9.2, output to dolby digital worked without issue. Since updating to Roku OS 9.3 (9.3.0 build 9149-30), I get dolby digital output when the TV first starts, then after a while it reverts to PCM stereo output. Going to Settings > Audio > S/PDIF and ARC, then changing from dolby digital, to PCM-Stereo, then back to dolby digital fixes the problem temporarily. Going into power settings and forcing a TV restart also restores dolby digital temporarily.
I haven't seen any resolutions reported for this bug.
This same issue was also discussed in the Roku reddit:
I got this to work on my system - Roku stick on a Vizio TV (no external speakers or soundbar).
On the Roku stick menu, go to Settings -> Audio -> HDMI -> and set to PCM - Stereo. It appears that the auto detect routine has a defect in it where it fails to switch over after one of the states was selected. The Audio Mode setting did not make a difference. This means no Dolby output, but at least I have sound.
Hope it works for you while Roku find the bug and squashes it.