Update:

I've changed my version to latest experimental and the bug still persists.

The bug only happens when trying to cross a gap of at least two spaces, with the last space being an underground belt entry/exit.
All belts in the picture are dragged from left to right. The middle are just placed ...

This bug is probably already reported but I could not find a post about it so I made one.

When dragging blue express belts over underground belts, the underground exit belt will not be placed.
This bug only occurs when using blue express belts.