Disabled fields updated via ajax events will not get saved on new records

I have a single record form with some currency fields disabled on insert\update mode.
Via ajax onChange events of other fields, these disabled fields get populated with calculated values.

This issue happens only on insert of a new record: when I press save, the calculated values in the disabled fields are not saved.
Everything works as expected If I open the form with an existing record and edit it.

In another and more complex form, the ajax triggers fields are in a detail form and the calculated fields (still disabled on insert\update) are in the master form.
In that form the issue is not present: calculated values in the disabled fields are saved on insert new record.

So what can be wrong with the simpler form (no master\detail) of my previous post??
Sometimes SC has really strange behaviors…