This is where the Changed Product Rule can get really messy. CPR basically says that parts of the aircraft that are unchanged from the previous model don't need to meet the latest regulations (assume they met the previous regulations at the time of the previous cert). But reasonable people can disagree over what constitutes a change. No first hand knowledge, but I suspect Boeing took the position that they weren't messing with the wire routing of the stab trim system - hence it was unchanged and they didn't need to meet the 2007 rule change - and since the MAX got certified the FAA must have accepted that. Now, the FAA is taking the position that MCAS makes the stab trim a changed system, and so the entire system - including wire routing - needs to meet the latest regulations.
I got caught out by this when we did the 767-2C (KC-46). My position was that the FADEC software was changing, so we had to recertify the s/w to the latest rules - but that the hardware was unchanged and hence didn't need to step up (the major difference being that the FADEC HIRF/Lightning had originally been certified via a Special Condition - now there are regulations for HIRF/Lightning - the Lightning regulations are pretty much the same as the SC, but the HIRF requirements have changed significantly). The FAA came back and said 'No, the FADEC is a changed system, the entire system needs to meet the latest regulations'. This resulted in big stink because it would have meant Pratt needed to re-test the FADEC for HIRF/Lightning (which is a seven figure expense) - even a North East FAA "National Resource Specialist" got involved - he thought the Seattle FAA wasn't interpreting CPR correctly.
Eventually we certified to the latest regulations - by taking credit for all the extra shielding we were adding to meet the military requirements for HIRF/Lightning/EMP.