Problematic electronic volume control TC9176P
The TC9176P doesn't determine what the volume should be - it just sets the volume to what it is told to set it to.

You want to be focusing your attention to the circuitry between the +/- buttons and the TC9176P.  This circuitry will be generating a serial bit-stream from some register values which are controlled by the +/- (incremented / decremented).

It sounds like either the data stream is being generated incorrectly or the register values are not being maintained / set properly.