Here what I get from dbus-monitor while clicking the play/pause button:

method call sender=:1.54 -> dest=org.freedesktop.Telepathy.Client.TelephonyServiceApprover serial=918 path=/com/canonical/Approver; interface=com.canonical.TelephonyServiceApprover; member=HandleMediaKey
boolean false
method call sender=:1.120 -> dest=com.canonical.TelephonyServiceHandler serial=300 path=/com/canonical/TelephonyServiceHandler; interface=com.canonical.TelephonyServiceHandler; member=HasCalls
method return sender=:1.49 -> dest=:1.120 reply_serial=300
boolean false
method return sender=:1.120 -> dest=:1.54 reply_serial=918
boolean false
method call sender=:1.120 -> dest=com.canonical.TelephonyServiceHandler serial=302 path=/com/canonical/TelephonyServiceHandler; interface=com.canonical.TelephonyServiceHandler; member=HasCalls
method return sender=:1.49 -> dest=:1.120 reply_serial=302
boolean false

Since switching back to UBports on my MX4 (after using CM13), I can't sync to my MTP device, after some files copied, I get this:
gvfsd[1223]: Received event PTP_EC_StoreAdded in session 1

It worked on CM13 and Ubuntu Touch last official OTA.

Any idea?