The solution was provided by JTAC.
This is a known issue that is _maybe_ solved in 15.x
https://prsearch.juniper.net/InfoCenter/index?page=prcontent&id=PR860226
Solution: Add all interfaces under stp. for example set protocols vstp interfaces <interface-name>.
Otherwise you might get random issues that the interface isnt learning MAC = no traffic is forwarded.
//Rob