Distance field shadowing / Foliage culling mess

Hello, we have a couple of problems with our game (open world).
We set an high distance field shadow distance to have far shadows but maybe we set it too high because I noticed that with higher values the terrain/foliage culling will totally mess up.
Is it normal? Is there a way to avoid that? This is so odd, the frame rate goes from 60 fps to 28 looking at the ground in a specific area. If I lower the distance field shadow distance to the preset everything it’s ok (fps drop is about 5-6 fps however).

Another problem is the foliage culling on tiled landscape.
We are experiencing serious problem with it. We have a tiled landscape, and at the moment we are painting foliage on the persistent level and not on the tiles. Is it correct? Or do we have to paint foliage on the tiles? Because I noticed (using the freezerendering command) that sometimes the culling is not correct: I mean, if I am looking to a desert with foliage behind me, if I use freezerendering turning around, the landscape tiles behind me will disappear, while the foliage is still rendering! This is a serious performance problem, and I hope that there is a way to avoid that.

Any idea?
Thank you very much!