Now that I had time to read R80.40 upgrade manual it all makes sense.. MVC (multi version cluster) only supported with R80.40.. That makes the "glue" then between different kernel versions and CCP.. So R80.30 2.6 and R80.30 3.10 will never synchronise. ouch, hard lesson.
Would be nice though if this was stated in R80.30 3.10 release notes as I just assumed that both 2.6 and 3.10 versions support unicast therefore clustering and sync would work.
Basically we only needed 3.10 to improve NIC performance (RX-DRP) when using MQ and avoid 5 interface limit with it.