Syno has several different historical methods of building and naming LVs.  I think this one must have been built on 6.1.x because it doesn't use the current design.  The current one has a "syno_vg_reserved_area" 2nd LV when a volume is created.  But yours may be complete with only one LV.   It should not be necessary, but make sure you have activated all your LV's by executing sudo vgchange -ay   At this point I think we have validated all the way up to the filesystem.