Just to let members know. Mazda have identified that the I-Eloop system DC/DC Converter is a likely cause of the symptoms discussed in this thread. A new DC/DC Converter has definitely cured the problem on my car but at great expense to me (£840 for the DC/DC Converter alone + labour).
I have managed to obtain a DC/DC Converter from a 2020 registered ND that has been written off and broken for spares. Details of the unit can be found in the ‘Buy and Sell’ section of the Forum at Mazda at ND (1.5 or 2.0 litre) i-eLoop DC/DC Converter DJ4V-67Y6Y-Revision C For Sale - Buy and Sell [Replies by PM System Only] / MX-5 & Roadster Parts For Sale - MX-5 Owners Club Forum