There are quite a few strange anomalies in floating foliage, several different fixes, but they don’t always work, so I wanted to share a recent find in a personal project in case this might work for someone else too.
I had previously attempted the usual RemoveLandscapeXYOffsets & grass.FlushCache, rebuilding grass maps (which at other times have worked before converting my project to UE5), etc. but continued to have the issue. I disconnected all of the sample/grass types within my landscape materials, saved, recompiled, etc., committed to source and then reopened the project and reconnected the grass types in the landscape material, compiled, and huzzah, they seem to be staying on the ground, without any random sheets or dotted foliage in the sky! 
It appears the disconnect of grass types and reconnection after recompiling and saving seemed to do the trick in this instance.