When something like this happens, it's usually caused by either a MIDI device (probably the MFC-101) sending MIDI data that's turning down the parameter, or by the Axe-FX having a parameter that's tied to a CC# that's not there. The fact that you get sound when changing the source to "none" confirms that this is probably happening.
Make sure your expression pedal is still sending CC7 messages.
Also make sure your MFC isn't configured to send a CC value (such as "0") when you change presets.
Also, backups are always a good idea.
Make sure your expression pedal is still sending CC7 messages.
Also make sure your MFC isn't configured to send a CC value (such as "0") when you change presets.
Also, backups are always a good idea.