Since the standard pak64 release 124.1.1, I remark the vehicle window behaves funny.
After changing any vehicle's dedicated line in the 'schedule' tab of it's individual window, and then clicking on the 'freight' tab for instance, the schedule tab will automatically pop right back up and the vehicle come to a halt again, see the video snippet.
At this instance also the before chosen destination sometimes automatically will be "randomly" changed.
After then clicking on the 'freight' tab once more, the vehicle finally just realizes the line-change and rides on.
This is true for no matter which type of vehicle. Also seems independent of any particular pak.
Oddly, seldomly the concerning vehicle after this procedure won't move anymore at all and get's stuck in place for ever until, again calling up it's schedule tab and closing it, after which it will move as expected. When unnoticed at the instant, the bus/truck/train happily will block traffic for ages :D
While blocked, the mouseover tooltip on the vehicle will read 'schedule changing', but nothing happens.
I wanted to report this last summer, as the V124.1.1 was current, but later forgot. Since I lately started to play Simu again, I still remarked this very behavior in this latest stable revision, r11590.