Actually they can if the seatime letter says voyage/route of the vessel. I remember the brief time on a SubChapter M vessel that the seatime letter mentioned the voyage/route(Louisiana, Caribbean, South America). Recently while being onboard a SubChapter L it swapped to SubChapter I, which they gave me 8 hrs days event though I was working 12 hrs. So I’m assuming the NMC evaluator gave him 8 hrs instead of the 12 hrs thinking 4 hrs as OT. At the end we are not NMC evaluators, we are just speculating and exercising our first amendment.
As for OP would just have to keep working or keep arguing with the evaluator. In my case I decided to keep working.