Field not updating

For a disabled lookup field ("Problem Field") I set the value through Deluge script in the 'On User Input' event of another field (on the same form). So depending on the value selected in this field, the value in the Problem Field changes.

This works fine for existing records. On new records the value remains "-Select-" until I save and reopen it.

My app bmpm is shared with support (edit access).

To reproduce:
- go to tab 'Admin'
- open view 'xxxx'
- click 'Add'
- In field 'Player' select the entry which starts with "Bern..."
-> the field 'Introducing Scout' still contains '-Select-'

Note:
I included a line in my code which entered the contents in Introducing_Scout in another field to see if it actually was empty, and it was not (otherwise the field would not be visible also).

Regards,
Filip