I am having the same volume issue with my TCL Roku 75R635.
I just bought a new Vizio 5.1 surround system with a soundbar, subwoofer and 2 satellite speakers.
As soon as I hooked it up and tested it to see if it was working, the volume started increasing and decreasing in increments of 2 when I adjusted the volume and the volume seems lower than it was with my older surround system even though the one I just bought is the newest, up to date surround system. With my old surround system, the volume at level 3 was sufficient to hear the program I was watching, but now the volume needs to be at least at level 6 to get the same level of sound whether I use the TV speakers or the surround system, I get the same results.
I have tried everything I can think of (except factory resetting my TCL TV since that would be a pain since I would lose everything, settings, account, etc), but nothing seems to correct the problem and the problem persists whether the soundbar is connected or not connected.
I even tried factory resetting the surround system, disconnected everything, shutting off the TV and starting from scratch, but it did not work. I also have tried with the TV volume both "On" and "Off" while using the surround system, but that doesn't work either.
Funny part is this new Vizio surround system replaced my old Vizio surround system and I never had this problem with the Vizio surround system I just replaced, the volume performed normally and seemed louder at a lower volume level. This issue just started after I connected my new surround system and like I said, it increases and decreases in increments of 2 even when the surround system is not connected and it does not matter whether I use my Roku remote or the surround system remote.
I hope someone figures this out soon as it is quite aggravating and very annoying!