I keep the "miter corners at 45" checked, but diagonals appear at random.
Can anybody tell me what's the rule behind this behaviour? Because if the diagonals were missing completely I would say
that they are not implemented, but in practice there must be some situation in which the routing engine actually draws a diagonal
or optimizes two orthogonal segments with a diagonal ... just understanding how it's implemented would help, maybe if I
dispose the components someway, or use some settings instead of others I can get more diagonals automatically ....
Thank you!

Mitering corners doesn't mean diagonal routing. It means 45-degree transition between two orthogonal segments. It's like a chamfer from mechanical industry.