Happened to me as well as soon as I dropped my payload near the bridge.
Blue 3 and 4 were both there but as Sulaco said as flight leader turns to engage the ground troops at his 3 o'clock, numbers 3 & 4 go a bit squirrely and turn left instead.
I'll try the mission a second time and see what happens.
UPDATE:
This was a regular issue for me.
In the end I found the best solution was to jettison my bombs soon after take-off. The mission played through just fine.
These missions are challenging and beautifully crafted. Another superb campaign from Reflected.