Values selected using multiple select fields are not passed to callback functions

I have a CRUD with two NtoN relation fields added using setRelationNtoN() function.

The primary key of the CRUD table is not an auto-increment one. I’m setting the primary key manually using the callbackBeforeInsert() function.

When I add a new item from the add form, data is added to all tables. But the foreign key fields in the relation tables are always inserted as 0.

Then I tried to use the callbackInsert() function to do the insert process in my own way. But the data selected using multiple select fields are not passed to callback functions.

How can I get values selected using multiple select fields from callback functions?