I have the same problem with my last fuel-up. The one before was a partial but this one was full so it should show mpg but no dice. And yes the last two entries are marked correctly.
My entries so far
1 - first fillup recorded - no mpg shown of course
2 - second fillup recorded and mpg is reported correctly
3 - partial fillup and no mpg shown which makes sense
4 - full fillup recorded but no mpg shown as it should be
Also, the desktop interface is reporting the same thing as the iOS app.