My custom stats also disappeared when website upgraded. Also, when I re-add them some odd things happen. I added "miles per G" for "last 3 months" and "last tank". Each of these showed zero and did not print the time frame (eg. "last 3 months"). Also, I added "MPG" for "last 3 months", and while this did print the "last 3 months", the value was obviously wrong.
I haven't checked whether the stats go away after logging out, as has been reported here.