Same here but with Audiophonics I-Sabre ES9028Q2M DAC.
It seems Volumio has changed the way to manage Mute in version 2.502.
In previous versions, volume level was set to zero and mute was set.
In current version, volume level is no longer set to zero. Mute is set only and does not work.
Alsamixer Mute is working fine. So the issue seems to be in the forwading of mute command from Volumio to Alsa.