The volume is indeed dropped to -3dB when turning on the channel link switch. According to the engineers this is due to the fact that the norminal position of the panpot is different between channel link on and off.
When channel link is on, the norminal position is “turned left or right” position. If you turn to center, volume is decreased from 0dB to -3dB.
When channel link off, it is “center” position. If you turn to left or right, volume is increased from 0dB to +3dB.
If one sets the panpot to left and right before clicking channel link switch, volume will be returned from +3dB to 0dB (norminal level) when channel link is clicked.
BTW, the behavior of the channel link on the MR and UR series is the same as the default settings of YAMAHA digital mixers such as CL, QL and etc…