What it looks like: a train passes a chain signal with two exists of which one is green and the other is red and the stops at the red one.
In pictures:
The trains is going to this station in block 748 after the selected signal from 745 to 748. The station is occupied and thus the signal is red
The train passes this chain signals, which is fine if it goes to the waiting bay with the now yellow light
The train does not go to the waiting bay and stops at the signal from 745 to 748
Now, what actually happens.
While the train passes the chain signal, it solemnly swears that it will go to the waiting bay
As soon as it has passed the chain signal, it breaks its oath and goes to the signal from 745 to 748
Maybe not 100% a bug, depending on at which points a train is allowed to recalculate its path in factorio logic. If it is not a bug, consider it a feature request. (For the following feature: do not recompute the trains path after its tail passes a chain signal.)