The bug would seem to be that LilyPond is not allowed to increase the
length of a stem to accommodate StemTremolo grobs with notes that are
beamed. Or perhaps, it is allowed but is failing to do so. Either way,
the code that expects to have enough room incorrectly computes a
degenerate interval, which eventually results in the failed assertion.
Not knowing a single thing about Lily's underlying code, I nevertheless agree that it's something like this. Lily *also* does not extend stems on un-beamed notes, leading to collisions between flags and tremolo glyphs.
Cheers,
A