Thank you for your reply 
Happens in a newly created project too, so it is not specific to my stuff. (so you can replicate it like this.)
What I did:
1: Start 5.1PV1
2: New Project → First Person (Blueprint, no RayTracing, Maximum Settings, Desktop)
3: Wait for compiling
4: It is immediately visible, since one of the 4 walls does not receive GI
5: I increased the sunlight to 10 and rotated it a bit, so that the “dark wall” is in shadow but should be lit by GI and placed another wall in front of it to get bouncelight.
6: saw this:
It only happens in one direction - the side of the wall that faces towards Y+ has this issue.
Lumen Scene also is fine, its just not being “rendered” properly in the final composition of everything.
Unrelated to Lumen, but still relevant: 5.1 does not work with amd drivers from earlier this year, unlike 5.0 - after a few seconds it crashes with “d3d device lost” or so. Not sure if this also happens in packaged builds. (If so, that might not be a good foundation for releasing anything with 5.1, unless fixed.)
Nanite also isnt working properly, some meshes are not displayed fully, but still cast their full shadow.
EDIT:
Small meshes going through trees cause a lot of overocclusion:
^^these are practically 1cm wide “cables” (4 of them, one for each corner) holding the whole thing, if I move it aside the overocclusion moves with them.
Something else I noticed: everything got brighter than it is in 5.0 (guess we have more bounce light now?), and trees obviously need to be adjusted to look fine again. (5.1 is not a simple “drop in replacement” for lumen, work will be required to adjust things.)
But I also see improvements with “dark spots”(not on trees, regular geometry) that existed in 5.0, if that is not another bug with AMD. Aside from that, Metal now seems to not be black anymore, especially if instanced.
From first impressions, good step forward - but given my “amd issues” I have a hard time to see what is real, what is a bug.
EDIT2:
The change to SM6 from SM5… that has no impact on system requirements, or does it? (hard to find information on what gpu supports what, with which driver etc.)