Sorry, I meant to post this sooner, but I've been indisposed the last couple of days.
I don't like the change in underground belts and do believe it is objectively a minor change for the worse. The old design, bug or not, had some advantages, like elegance, compactness and it was easy to understand. Additionally, the new change will break my whole mega base and it also widens the gap again between belts and bots.
The first point of elegance is very hard to quantify, but I do believe the old solution was more elegant. It also looked good and I started to use it everywhere, even when not needed, especially with mods. I even use it in my mining rows, together with a pole and a light in each hole.
Without this trick, we either sacrifice the elegance or the compactness of our builds. I wanted to explain this in words, in a lengthy paragraph, but enigma888 came to the rescue. He likes the change and illustrated a solution I wanted to suggest too, but used an animation instead of words. In this animation, we can see an alternative that first of all works correctly. Second of all, is IMHO less elegant, more noisy/denser and compared to adding a couple of underground downright clunky. Doing that is just busywork. Nobody want to worry with such stuff, we just want a saturated line. To be fair, the underground solution was busy work too. Ideally, inserters would be able to saturate lines on their own, but we need tricks for now. The smallest less busy work trick should win.
And it doesn't work with beacons. As far as I'm concerned, the pre-beacon phase of Factorio is probably the greatest and most fun mega-tutorial for building a mega-base in the whole history of gaming. But only with beacons will you have you final design for a mega-base.
With the old design, we needed exactly 7 smelters on one side to get a fully saturated blue belt. It relied on heavy underground use. It will take a very long time to come up with a new elegant design for this that is the smallest possible. After that I need to redo my entire smelting setup. I don't use a main bus and do not cross balance my smelting lines, because I use "better" modular designs, where everything is spreadsheeted and designed to perfectly accommodate full belts of input and consume them all. Even the smallest hole in my inputs will ruin my base.
Finally, underground belts are very easy to understand. Put two down. That's it! In 2 tiles you understood 100% of their complexity.
On the other hand spliters are more complex and have so many hidden properties. As a testament that they are harder to understand, you can take many random base, even from experienced players, and I'm sure you will find some redundant balancers, that actually do nothing. Or lopsided balancers, that do not fully equalize their not balanced inputs into s single output as the one who placed it expects to. Or if you need further proof on their complexity, do throughput analysis on various 8 by 8 balancers. See how easy it is...
If this change is final, I will surely accept is and continue playing Factorio, which is probably one if not my most favorite game. But I'll only update after I'm done with my current bases.