Image array upload > ACF not working

Hello,
I’ve set up a form for front end Portfolio submissions

I have Portfolio custom post type with 2 custom fields (generated bu ACF). There are :

A color picker field (title_colour)
A gallery field (job_images)

In the Forminator Form I did the following:-

Added upload field (upload-1)
Added input field (text-1)
Added PostData field (postdata-1)

In Postdata field I mapped 2 custom fields:

job_images > upload-1
title_colour > text-1

The form submits without error.
A Portfolio post is created & the uploaded images are added to the media librarycorrectly. However the job_images (gallery) field in the post is left unpopulated.

Any tips gratefully received!