I did some more testing in an empty 5.8 and empty 5.7 project. I am able to recreate the missing planar reflections and also get a few bugs when it comes to the planar actor:
Right is 5.7 and left is 5.8 and already can see issues.
I didnt chance anything except for placing a planar actor and the 5.8 version gets heavy flickering and a weird shadow in the reflection. 5.7 is completely fine.
And with a post process volume, unbound, only GI on Lumen and reflection on NONE:
Again, in 5.8, no planar reflection at all. I know that the reflection in 5.7 are from the planar actor due to the small distortion in the lower part, which can be corrected via ExtraFOV from the the planar actor.