I'm not sure if this is the same bug.
In your report you try to place belt facing opposite site than the splitter.
In my case the belt faces always the same direction as the splitter but the behavior is different depending on the side from which belts are placed.

The bug is still present in 2.0.60

Dragging belt through splitter if there is already belt in the same direction causes a underground to be placed if the belt is on the other side of the splitter.
The new bet is facing the same direction as splitter and existing belt.
See attached video, row 4 and 5 (count from 1 on top).

I would ...