This is not a bug, it’s intended.
If you look closely, the planned extension of the bridge to the side doesn’t show you visually, how the bridge pillars are looking after the extension of the bridge to two trucks, but the game know, that the pillar would stand in the double track below the bridge, so it says “Bridge Pillar collision”.
Either split your above track before the below one so you build two seperate bridges and reunite the tracks after wards or redo the whole bridge.
I made the experience, that you get the best results with bridges crossing over roads or tracks, if they cross in a 90° Angle.
The more flat the angle is, the earlier it will make problems, either in preventing the bridge extension or in not building the bridge at all.