(I am quite sure this is a duplicate, but I still wanted to record this just in case it is not.)
In the release notes for 2.0.40, there is an entry reading "Fixed another bug related to smart underground belt building." There is no bug ID to go with it, so I'm not sure if the intent was to fix the issue described in this post.
There is still a rather annoying bug remaining in vanilla, unmodded 2.0.41 whereby a ghosted pair of underground spanning any entity will fail to be built when dragging real belts across the underground-entity span. For example: If there is a ghosted underground belt immediately to the left of a real splitter, and to the immediate right of the splitter, the underground belt's pair, dragging belt in the direction of the underground belt will result in only the first underground belt tile being built, not both.
It is probably better to explain in video than in text, so kindly refer to this clip.
If this was fixed for 2.0.40, then there is a regression for 2.0.41.
[2.0.41 vanilla] Underground belt bug
Re: [2.0.41 vanilla] Underground belt bug
Attached is a screen capture of this behavior in 2.0.41 with no mods installed:
It happened before the update and it's still happening after the update.
And here some screen captures from a few days ago (probably 2.0.39 or 2.0.38) with mods installed, captured during actual gameplay. The problem is worse (the second undergorund belt is sometimes placed even further away than it needs to be) but I can't determine whether that's because of the mods, since it didn't happen during testing without mods, and I don't actually play without mods regularly. But I figured I might as well add them anyways.
And here some screen captures from a few days ago (probably 2.0.39 or 2.0.38) with mods installed, captured during actual gameplay. The problem is worse (the second undergorund belt is sometimes placed even further away than it needs to be) but I can't determine whether that's because of the mods, since it didn't happen during testing without mods, and I don't actually play without mods regularly. But I figured I might as well add them anyways.