oing back to 25b from 26a removed half of things that been build

I went back to 25b after encountering too many problems

What I expected to happen to find my world the way I left it when last logging out
What actually happened was I "got set back a couple of weeks progress wise". for example: the whole storage room I basicly finished building was half gone and whole areas needed redoing.

When I went to 26a again, it was as it was again.