Ok, here's what I've found...loading "high" rules in the advanced mode, removing the USSEP mesh rules for the trees and using ultra lod did result in most of the trees behind the Winking Skeever remaining without vanishing when approached. There are still some trees further back behind these, though, as well as some behind Castle Dour that do still disappear when you get within a certain distance. When I discussed this issue with Mousetick a couple months ago, I also mentioned how 3 or 4 mountain peak/ridge/cliffs (also behind Castle Dour) had no lod representation when approached from the Blue Palace and would suddenly render into place. I'm guessing these other objects weren't addresses by USSEP (at least not the version I'm using which is the last one for 1.5.97) Anyhoo here are my logs:
No bugreport generated
Dyndolod & TexGen logs: https://ufile.io/f/kj4qh
TexGen debug (there was no Dyndolod debug generated): https://ufile.io/rw2kbfzg
I hope this helps, I really appreciate it!