I'm having a similar issue on my 2014 LX570. Checked all fuses, all good, ran diagnostics, no codes/failures. I've reseated all plugs and connections within the dash, reached under the passenger seat to the RSE module to do the same.
Does yours show volume adjustment on-screen?
Mine does not
Does yours show audio adjustments (balance etc) in a centered fashion?
I've seen some situations on the web where an amp failure showed those levels maxed (all the way to the right and rear) and not adjustable. Mine shows centered.
I've seen multiple people mention United Audio doing repairs on the amp, but also have seen several posts mentioning the amp repair did not solve their problems. Similarly, the RSE module (multi display controller? under the passenger seat) seems to have similar symptoms when it fails, but I have not turned up much on resolving this failure.