Review Heatmap lost in 2.1.56 qt6

Hey there,
I use Windows 10 and just updated to Anki 2.1.56 qt6.

Until then the heatmap always worked well. All the days I learned were green. But now it stopped working. Here is a sample screenshot of how it looks.

Is this normal when upgrading and the add-on maker will make an update or should I go back to Anki 2.1.54 ?

image