111 lower arm mirrored


I got some artifacts after using boolean modifier while using soft shading. When I turn the flat shading the artifacts are not there. What is going on? Maybe I’ll remake it to something more simple while this problem with artifacts is solved, but being honest I’d like to save the current design. Please, advice? Thanks.

Ok, I googled and found a solution to artifacts with smooth shading it is “split edge” modifier.
Before:

After:

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