Error in validity (report) (was: multipolygon buffer result either invalid geometry or valid geometry but wrong)

I want to buffer of a small amount the multipolygon in the following picture (I apologize if it is quite complicated, I tried to replicate the issue in a simpler shape but I could not make it):


When applying the buffer algorithm, I have encountered two problems:
1. if the buffer amount is 0.05, we obtain an invalid geometry due to self-intersections in the external border;
2. if the buffer amount is 0.050005, we obtain a valid geometry, but the result is wrong (see image below).

Here below a godbolt link to replicate this problem:
https://godbolt.org/z/vM9ex7MrW