Thanks for the update....it's too soon to say that's it....10 years ago it was routing to close to the exgaust...then it was the master cylinder.
...fun fact, the monster 821 shares the same brake assembly as the multistrada and m1200....all with their own issues...all with different assumptions...none have really worked...to be fair...brake lines are the only thing left....was going to just replace them all...but turns out there is no m821 master cylinder to abs replacement from the factory for the 2015 model
Welp....to throw another spanner into the mix...I've switched to a high temp brake fluid...motul rbf600....about the same time ago...it's still as taught as it was the day I did it....even with the little riding I have managed...while I'm incredibly glad for your outcome; I don't believe it to be a blanket cure-all for everyone or perhaps even the next fan theory....although I am curious what fluid they used to fill your system....now I haven't ridden much this year....hardly at all honestly...and with the temp dropping I'm not going to be able to test any temp theories....but I am thinking of using an Arduino with some temp probes to register some real world data points....in an attempt to put this topic to rest...if it was one thing it would have been sorted years ago....and my experience with mushy brakes after the abs engaged led me down one path...then after sitting still...mush..It’s been about 1.5 months since I’ve change rear break lines. It solved all the issues with rear brakes. It keeps the same pressure as it was right after bleeding.
I’ve also changed the rear brake cylinder but it didn’t make any difference. So I suggest anyone coming across this issue change the lines first and check if it works before going any further.
Shame on Ducati they are not taking responsibility for their suppliers and defective parts.
In case someone ever come around this problem, you got to take apart almost half of motorcycle. Had to leave my bike in service for two days.