... And will often reset to a stage of progress before you have attached chambers to a nuclear reactor
the inventory is saved properly and up to date but not the world itself
strange things can happen like on chunk reset and another untouched (my reactor containment chamber got cut in half by sand)
this will happen if you have changed anything in the world and "save and quit" then reload the world later
The world does not save properly...
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It's a known bug and is already fixed in the next version.