I've been experiencing a similar issue. However in my case, 7 out of 8 PVE islands spawn bosses, 1 does not, no matter what configuration(s) I've tried.
Over the past few weeks, I've set up numerous test environments with varying server/island/configuration/setting differences, all with the same result. (7 out of 8 PVE islands spawn bosses, 1 does not) In every case, the outlying PVE island has been Mnt_R_CL_PVE
Over the coming weeks, I'll be rebuilding my 2x2 cluster from scratch and will also try experimenting with a 2x3.
More to follow.....