Disable validation of a descriptive flexfield using forms personalization

Hi all !

I've defined a DFF where some segments are required and some are not.

But in one special case, this DFF is not supposed to be filled in at all.

So, I made the following personalization :

But it had no effect. When I save the invoice, validation complaints that there are missing values in the DFF.

Is it at all possible to force validation status on the DFF so that it surpasses validation ?

I also tried to do the same in custom.pll :

IF (event_name = 'WHEN-VALIDATE-RECORD' AND form_name = 'ARXTWMAI') THEN
app_item_property2.set_property(
  'TGW_HEADER.DESC_FLEX_MIR'