This may not be much help but hopefully will point someone in the right direction. I have a Z83 board (Atom) and had the same problem with a rt5651 device (though I started on 18.04, did not try 16.04). In that case the manufacturer’s site had been recommending code from here https://github.com/plbossart/UCM.git, which apparently worked at one time (so maybe 16.04). The code there worked, but caused mono not stereo output. I found that the plain distro code in 18.10 (maybe 18.04 have not tried that again yet) worked without modification.
My point is not to try 18.10, but rather that I think some code has been floating around possibly since 16.04 that in 18.04 causes mono output. Sometimes. At least for me. You might investigate the code in the vanilla distro (look in /usr/share/alsa/ucm perhaps) against what’s in the pi distro you are using. Also if present asound.state in /var/lib/alsa.
I did not try to figure out what was wrong in mine, I just stopped using the flawed code.
Again, this was Z83 not Pi, so a lot of differences, but offered FWIW. And so you know you aren’t alone.
Linwood