Editing Test Step User Picker field saves mismatched accountId, shows persistent "Invalid User" on Cloud

Description

Editing the Test Step User Picker field on an existing Test Step and saving a different user results in the field permanently showing "Invalid User." The field works correctly the first time it is set on a new Test Step; only editing an already-set value is affected.

Pre-conditions

• A Test Step custom field of type User Picker is configured.
• The Test Step already exists and has a value set for that field (bug only reproduces on edit, not on first-time creation).
• Confirmed on Xray Cloud; customer confirms the equivalent flow works correctly on Jira DC.

How to reproduce

1. Configure a Test Step field like the image below:
   
2. Create a test step and fill out the field, adding a user to the User Picker field.
3. Save the field.
4. Reopen the test step and edit the User Picker field, selecting a new user.

Actual result:

The Step Owner field displays "Invalid User" after the edit is saved and the page is refreshed.

Expected result:

Changing the Step Owner to a different valid Jira user and saving should persist that user's correct accountId and display the selected user's name after save/refresh, the same as it does on initial field creation.