I’ve setup a similar test case using the third person template (just because that’s what I had loaded up at the time) with a wall I had made in 3Ds Max. I setup a closed box, similar to your setup.
I tested this with two instances.
For my setup I did apply the m_Metal_Chrome material since I know it’ll produce a reflective surface.
I used a static directional light with light built on Production and got no light bleed, but did when building on preview (this is expected).
Then I switched to Distance Field Shadows by setting my cascaded shadow map distance from 20k to 0. I enabled DFS and left at the default settings. Rebuilt lighting and I still have no shadow bleed.
Can you test this setup as well and if you’re still having the same issue can you post a screenshot of your directional light details panel?