This is the latest autosave that I have. The save before the error was already overwritten after I tried to replicate the situation few times, without any success. I have 15 minutes autosave window and the crash occurred ~10 minutes after last save. Situation has not repeated itself yet.

As I do ...

I do not have exact steps to reproduce but I will try my best to help this great game.

1. What did you do?

New save file started on 2.1.7, light in mods but beside few loaders nothing was placed / used during the crash.
I was reverting underground belts using map view.

2. What happened?

Game ...

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 ...