Michael,
Just made the flight from Tunis to Monastir ( just 20 minutes) with the Arc of 13nm programmed into the FMC.

It flew the Arc like in reality.

A small issue I encountered : when the aircraft made the turn from the last point of the Arc and came out of Rinos ( +- 2700 ft ) it was a  to much to the right of the Localizer.
I had to let it fly 60 degrees , while descending to 1900 ft, untill it captured the glidescope at 72 degrees.
If I had the aurcraft not descending manually it would have overflew the airport.

I remarked that you made your Arc at 20 nm.
Maybe a wider Arc will solve this.
Today I will try with a 15 nm Arc.

That last peace of the puzzle I need to solve too... ;)