I ran into this EXACT same issue: double trees, branches/canopy disappearing as you get closer to them. In my case, it turns out this was caused by an option in a mod called "Morrowind Optimization Patch", namely "MGE XE Addon". After I disabled "MGE XE Addon" in the "Morrowind Optimization Patch", I had no more issues with disappearing/overlapping trees. More information here: https://github.com/Tyler799/Morrowind-2020/issues/144