My bad, I realize that there is an issue regarding inkscape as well.
In the meantime, I went back to Windows 10 again just to confirm my previous try. So, on snapping the squares, I saw no gap between them whatsoever while working on LO Draw. However, exporting them as .svg or .pdf would show a consistent gap. So, this would either be an export bug or a rendering bug. I guess, it could be that I use Skia rendering on LO Draw but my pdf viewer probably doesn’t?
Long story short, I am glad my understanding of how snapping should work is on point. And as you pointed out, there is more than just a single bug here: Rendering bug in Linux, Export bugs for .emf, .svg, .pdf files and Inkscape’s bug (which is not LO’s concern of course).
What is the most straightforward solution here? Are there other rendering options available for Linux?
Honestly, this problem would have been a lot easier to solve if gradient options were less limited. I don’t know why it was decided that 3 colors should be the minimum, while in Microsoft Office there is an option for 2 that works in LO, albeit without UI.