The correct sequence of events during the (M)ISSU from a software release which does not support UM MCS (old release), to a software release that does support UM MCS (new release) is the following:
The old release has ESM and Gx synchronization enabled but the UM does not. This is because UM synchronization is not supported on older releases.
(M)ISSU is performed
The new software release boots up with UM still disabled. UM must be enabled via Gx directives send from PCRF.
The UM can now be enabled in the new release. When enabled, synchronization automatically takes place.