@Cookie-On-Tour the Scenic app also uses HERE maps, and on a 10 day, 4000mi trip, where 2 people were using it, no such issue existed.
I am by no means a techinical expert in the operating systems of MRA vs Scenic, however, it sounds like there is a different algorithym between the two - is this the difference - I do not know. I've also read reports where the HERE We Go nav app suffers the same fate.
Until this issue is resolved, it renders the Nav app un useable for me because of the extreme battery degredation, the extreme heat generated by the phone continually trying to keep up the quick discharge, AND the issue is exacerbated by high ambient temps outside - for me, once the ambient temp is ~25 degC (80DegF), the phone goes into self preservation mode, and, well, end of Nav app.