The occlusion is not active in the scene, not from post-process nor on the lightmass settings.
I’ve been doing some tests with the meshes. The only way I managed to get rid of the issue is to keep perfectly squared planes and meshes, with no modeling chaos at all and only keeping the edges as alpha.
Here, the try with square alphas only:
So then I removed the bevel inside the plank, which looked like this:
And the final iteration, without anything at all (bare planes, square plank), works… but obviously it doesn’t look right.
So this means that it IS a mesh issue… but how can I adapt it to have an organic look without this problem?