Weird, I did some further testing and I noticed your file had a POPM value of 78.
When I set the value to 70, MB shows 3,5 stars.
When I set the value to 80, MB shows 4 stars.
But using a value from 71 to 79 indeed results in 1,5 stars.
There is probably some logic behind that for the sake of compatibility with other software, but it looks a bit strange.