Hi thanks for getting back so quickly,
I double checked, and I have installed the latest versions of Tweakscale, the Uberpaket, redistributable, and steal back my fuel. This is Ckan though, so to make sure, would that be up to date, as in, is it still being supported on Ckan? I know some mod authors have been having issues. I checked and steal back my fuel is activated, this doesn't seem to be the issue.
I started a new sandbox save and the same issue persisted, made a quick craft with 2 FLT-800 fuel tanks, one of them scaled up to 2.5, fuel scales correctly, saved the craft, loaded the craft again in editor and the fuel is back down to the original amount. I wasn't quite sure which craft file to share, since there were three? This is one part of KSP I've never really messed with so I've just attached all three with the log file. The craft is just called 'test'. I do use quite a few mods and you're so right, a big bork like this would've been caught if it was an issue with the mod rather than my setup. Sort of weird though that its affecting the stock parts, if it is a mod compatibility issue. I'm not a modder myself so I'm hoping this is something dead obvious I've messed up so it's an easy fix, but lmk whenever you get the chance to take a look!
KSP.log
https://www.dropbox.com/s/ttov9d8735l7pi3/KSP.log?dl=0
Test.craft
https://www.dropbox.com/s/cm8sp3bhkv46xlp/Test.craft?dl=0
Test.craft.original
https://www.dropbox.com/s/spzyt4yfzzaikde/Test.craft.original?dl=0
Test.loadmeta
https://www.dropbox.com/s/rpos6g23nbprrmd/Test.loadmeta?dl=0