QGIS 3.10: forms_2 parents table/1 child table

I have 2 parents table (shape): one for trees (pk = id_tree); another for parks (pk = id_park).

I have 1 child table to list our works (fk = fk_works related to ‘id_tree’ and ‘id_park’).

In both cases, it’s a one-to-many relations.
I declared both the relations in the properties of my QGIS project.
I build my forms.
But I can’t add new work because, in the child table, the values of fk_works disappear.

I don’t understand why and what to do to add new work for a special tree or park?