When I setup a webform to update existing activities, and I give it cid and activity1 parameters in the URL, the webform loads up contact information and core activity information, but not the custom fields data. The custom fields just stay blank.

My first instinct was to re-examine the permissions, but this is the administrator account (with all Drupal permissions checked), additionally I added the Civicrm ACL permission to view/edit the custom field (not that this should matter) but the issue persists. I can add activities and custom data just fine, display the data using Views, but custom field data just doesn't load up in the webform, it's blank.

How would I go about debugging this? Am I missing something obvious?

I tried to recreate the issue on another instance with the same version of Civi and Drupal, but I couldn't do it, everything just works there.

Civicrm 5.20.0 | Drupal 7.72 (modules up to date) | PHP 7.1.33