Baked GI with modular assets problem

Hello there,

I've been searching for a solution for days. It seems to be a very common issue, although from dozens of articles and forum posts, I couldn't find any clear explanation for why is this happening, let alone any kind of solution to prevent it (aside from ones that would fundamentally alter my workflow).

Problem

I am using modular assets to build levels. My project requires baked GI due to performance expectations. However, whatever I do (or even if I leave everythng set to default), assets' lighting is never baked correctly. See picture below: What I would expect it to look like is something like this (using real-time GI and having mixed GI turned off). At first, I suspected my assets to be causing this issue as I have read something about this happening due to overlapping UVs (which my assets have, on purpose). Although when I tried to place generic Unity cubes, results were just the same. I tried to do the same in empty project with default lightmapper settings, also tried URP, there is no difference, each time, assets look like those in pic no.1.

So my question is - what is causing this behavior of the lightmapper? And how to prevent it?

I have read that some folks have fixed it using mesh combining tool that unified modular pieces into one. As I said at the beginning though, this is the solution I would like to avoid, because it would make the whole modular assets workflow kinda useless.