So I’ve retested and I obviously made a mistake first time around. Accept my humble apology.  

Seems that the static delta table does refresh with each microbatch. So the most recent version of the table is joined to the steaming df with each new microbatch. 

Really appreciate your help @szymon_dybczak. Thanks a lot. 

This has been bugging me for a few days and finally I have an answer.