None of the fertility increasing crops are applying fertility improvements to fields - regardless of other crops, number of workers or base fertility rating.
Composters add about 4% so seem to be working as expected.
Using Crops like Clover, Beans etc that should positively effect fertility, periodically have no effect on the fields fertility value. The only way so far that seems to resolve this, in part is to reload the save multiple times and hope.
I don’t know how to reproduce specifically, but it seems to be an issue more often than not.
Negative fertility modifiers are working as expected.