a train choosing the route before a one-way signal

[hovering teacup]

this happens when
1) the one-way sig in question leads within a certain distance to a branching, all whose branches start with a two-way signal.
2) all sigs between the branching and the one-way sig in question are red.
3) the wrong route goes in direction of the destination at the moment.

(the attached photo shows no one-way signal between the branching and the one-way signal in question, but it's the same if there's some.)

is it this way from the unpatched TTD ? or some feature of the patch changed it ?

[Patchman]

The patch doesn't alter train path finding. That is so complex that it's one of the few completely unmodified things in TTD, and will probably remain so for the future.

[hovering teacup]

OK
anyway now that i 've found how an occasional astray happens, it's easy to avoid.