get_field() with Group field not working anymore

Hi folks,

I've developed a Plugin long time ago. Now my client ask me to fix a bug. After some research I found the problem:
The get_field() function is not working anymore with a Group field.

global $post;
$attributes = get_field('attributes', $post->ID);
$price = get_field('price', $post->ID);
$attributes_price = get_field('attributes_price', $post->ID);
var_dump($attributes); 
// bool(false) 
var_dump($price);
// NULL
var_dump($attributes_price);
// string(6) "103,00"

I've also checked this behavior on another test site, but there get_field() on Group field returns an array of sub fields.

Any hint what is causing this issue?

Thank you!

Advanced Custom Fields PRO: 5.10.2
WordPress: 5.8.1

Start by reproducing get_field() with the reported Group field on Advanced Custom Fields PRO 5.10.2 and WordPress 5.8.1, comparing the attributes, price, and attributes_price results with the test site where the Group field works. Done means identifying why the Group field returns false or null and restoring the expected array of subfields.