I'm trying again, this time with a "05" value that is higher than even my first save in expedition had (it had "04") and this time it didn't revert, but it did drop from "excellent" to "satisfactory" very fast.
EDIT:
I'm now trying with "FF" value and it's also very-high, and hasn't dropped to satisfactory yet. I think this method actually works, as long as you do the edits on a savegame that is currently INSIDE the black sea.
EDIT2:
After walking around and sleeping once, I re-checked the flag values I had set at "FF" and they're now "FD".