It just occurred to me. Being a modern BMW engine and WLTP emissions and all that, I would assume you have a GPF - Gas Particulate Filter. All petrol BMW engines now have this. If I understand it correctly, the filter burns off the particulates during normal driving, assuming the car is taken at motorway speeds at times and for a decent distance. Assuming it has one, the CPU take over if it deems that the filter needs cleaning. Assuming it has one and being a Morgan they are normally driven on shorter drives vs. long motorway journeys, it could be it's going through a cleaning cycle.
Might be worth checking the manual for any mention of a GPF.
BMW GPFGPF FAQ
