Not sure exactly which version this was introduced with, but I only noticed after the latest release.
Also, not sure if this is intended behaviour. I have tested in the nightly and had the same result.
When a rail convoy uses a choose signal, it will enter the platform differently if it can access it's scheduled platform or if it has to choose another.
When choosing another platform, it will only enter far enough for the whole convoy to be in the platform. Whereas it will enter until the end of the platform if it is able to access the platform in its schedule.
I know there were some changes recently around this and some mentions of settings can be found in the last two messages on this post
https://forum.simutrans.com/index.php/topic,23763.msg211758.html#msg211758
For clarity I have also made a quick little test track to show what I mean and have recorded it in this video
This is indeed wrong. Investigating.