problem with the transparency I'm fairly new on blender (and I'm using version 2.8).
I have a strange problem with the transparency of 2 objects. The geometries are identical but are imported from 2 different .obj files. How come they behave like this? Even if I combine them with CTRL + J it's the same thing.

I thank you for any help.
Check normals, doubled vertices, and possibly triangles to quads.

Normals: Ctrl+Shift+N "outside".

Merge doubled vertices: Edit mode, select all, Alt+M "by Distance".

Triangles to quads: Edit mode, select all, Alt+J.