I don't think any "rule" here ever makes sense. It has to be done on a case-by-case basis.
The best rule-of-thumb for me is that if a server ticket's implementation requires discussion of edge cases, deeper questions of "do we want/need this complexity and what impact does it have on the dataset for end users, taggers and customers who use the structured data?" then that discussion should take place on Style and thus MAY lead to 1+ STYLE tickets. Thus the STYLE and MBS tickets can become dependent on one another.
It's natural though that the initial MBS ticket may attract discussion - although this isn't really the best place to come to a decision, if a decision is needed. The style process is the only proper, structured mechanism we have for resolving community differences - so if a ticket's implementation has trade-offs to be made, it needs to be done here.
To me this is similar to lots of governance processes in the real world. Someone wants to build a road. A company is asked to cost the road and prepare/plan for "implementation" (MBS ticket). Sometimes the road can just be built under existing resources and budgets. Perhaps it goes through existing public land with no sensitive environmental issues.
However sometimes building the road involves trade-offs and costs to other parties and there is a need for mediation and structure around making these tradeoffs. Perhaps a law change is required to allow use of the land. Perhaps building the road requires other systems to be set up to mediate use of the road (bylaws, use restrictions etc). This discussion might then need to take place in a government body whose purpose is to make decisions like this that impose winners and losers - this is like a STYLE discussion to me. Supporters and opponents of the road can argue their case. Sometimes the discussion makes it obvious that changing bylaws or guidance for the new road's use is required - this is like a STYLE ticket. But not always; perhaps the discussion comes to the conclusion that the road can be built under existing laws.
In the meantime, some or a lot of the implementation/costing/planning may have been done (MBS ticket)... but the two processes are often interdependent in any case.