On a custom module we have a One2Many that contains a Many2Many field on it. The Many2Many field has a default value which is correctly rendered on the client but after saving the parent record it's values are removed.
I debugged the issue an server side an I see that the Many2Many field is not send to server (only other fields). I think the problem was introduced with issue6754, which includes only modified fields but the default values are not marked as modified. I'm not sure how to mark them.
We've found this issue on tryton 5.0 but probably the behaviour is the same for sao.