The scenario is as the following:
Map a Salesforce Text field(Created by) with the Jira Reporter field.
For this field mapping, value map 2 Jira users. User A and User B. User A is an inactive user. The value mappings are:
User A (Jira) < Owner A (SF)
User B (Jira) < Owner B (SF)
In Salesforce, change the 'Created by' field value to 'Owner A' and initialize a push from Salesforce. The push fails and an error message is prompted.
Changed the 'Created by' field value to 'Owner B' and initialize a push from Salesforce. The push is successful.
Side note: If the user is inactive, it's not possible to search this user when doing the value mapping. During the test, the user was deactivated after the value mapping.