Custom fields using multi-select are non-functional after CiviCRM 5.31.0 update

Custom fields using multi-select are non-functional after CiviCRM 5.31.0 update. This seems to be related to this change, https://github.com/civicrm/civicrm-core/pull/18419

It appears that following the upgrade to CiviCRM 5.31, multi-select custom fields have not been updated correctly to use the new regime. They were all mapped to the newly described 'Single-line input field' (text field) instead of a Select list with multi-select option checked. On the custom group page, such fields continued to show as field type 'Multi-select', but were never rendered in display or edit mode on contacts or elsewhere.

Fixing this issue requires editing all of the multi-select fields and manually adjusting them to the correct new widget 'Drop-down (select list)' and checked the Multi-Select option. Ideally the upgrade process would ensure that these fields are correctly mapped when upgrading to 5.31.0

Agileware Ref: CIVICRM-1614