Dobro wrote

Thanks for the explanation, it sounds quite scientific, but I guess I could catch the core of it.

I've tried the other way as well, first trimming and then fillet, and the result is excellent.
Example_fillet.dxf

On the left there is the first attempt, where fillet is bad. On the right the second, where the line is uninterruptedly continuous.

So there's a solution for this problem, but one have to figure it out.