Can't find leakage, lbs man thought it was a seal/valve in the brake lever/reservoir giving way under pressure. Is that unlikely? It's how he explained the initial bite then instant fade in power...
I guess the obvious suggestion would be to swap the levers and see if that cures the front brake and kills the rear. Then you know you have a problematic master cylinder.
But what I faff to do so. I see your predicament when a pair of Deore brakes are supercheaps.
If it's a seal there will be evidence of leakage, somewhere.
But yeah mineral oil is less unpleasant stuff.