In my magazine and book projects, I sometimes fix pagebreaking by stretching paragraphs with \looseness=1

Unfortunately this works _in my project_ only with MkIV, i.e. \looseness has no effect with LMTX.

I tried different examples (e.g. the attached file) and can’t reproduce the problem, there MkIV and LMTX always behave the same.
So it must be something in my settings.

What could cause the difference? 
I tried English and German and different settings of alignment and tolerance.
Might it be font dependent?


Hraban