Hello,
Lilypond handles slanted Beams in a geometrically weird way, that is, the
thickness is not measured as the shortest distance between the opposing sides
of the boundary, but as vertical distance. This results in Beams getting
optically thinner and closer the higher the slope is. But we can very easily
factor this out by adjusting the thickness to the slope. In fact if we want to
achieve a real thickness theta the adjusted thickness would need to be
theta·sqrt(1 + slope²). See attached an experimental example.