Multiple updates are required today to update features. Features should be updated even when there is a link between parameters of different parts.
Not all features are regenerated when a relation is defined between dimensions/parameters of different parts. More specifically, ･Part A length: Analysis feature a ･Part B surface: Offset amount b ･Part B relation set to b=a When regenerated, Value b equals Value a internally, but this is not reflected on the surface location on the 3D model. When regenerated again, the surface location is moved to the correct location.
It's a great theory, but not possible. The nature of the software is sequential and that's part of its power.
Initial relations are evaluated prior to regeneration, post-regen relations are evaluated after. That covers most situations, but not all as your example illustrates.
Your example could be solved by making the relation b=a a feature relation inside the feature that contains dim B or perhaps in another feature in between the analysis feature and the feature that contains B.
I'd like to create a dedicated datum point through the default planes and attach the relations to it. Then you can name it 'dim_b_relations' or something that will help you remember why it's there.