Update matrix system form

Dear SAP experts,

I am trying to edit the matrix of a system form (Sales Order) through a bespoke Add-On.

I successfully managed to add and delete items, but for some reason, the totals are not recalculated. They still show the original totals before the update. I'd rather leave this to the system instead of doing it myself.

I tried to execute the Matrix's method 'FlushToDataSource()' but no luck (it gives me an error 'The item is not a user defined item 66000-8').

I also tried (naïvely) to refresh the form itself.

How can I achieve this and also to commit the changes to the database?

Using SAP B1 8.81 / C#

Regards.