This issue has been fixed by the release of 360° v1.6 and it's associated plug-ins:
SSL 360 v1.6.12
Channel Strip 2 v2.4.10
4K B v1.4.10
Original Issue
UC1/360° Plug-in Mixer with Pro Tools - Solo/Mute only solos or mutes the left-hand side of the 4K B or Channel Strip 2 plug-in when using multi-mono instances. The work around is to replace multi-mono instances with true multichannel instances (stereo instances).
A fix is being investigated.