Salesforce custom fields are not visible for mapping
When trying to add a formula field to the mappings, this custom field does not show up in the field-mapping lookup.
Jira cloud and server
When trying to add a custom field (formula), the field does not display as an option for the mappings.
This happens because the formula field has been created with a relationship from a Child Object and Child Objects are not supported by the connector.
Formula fields with relationship configured can be detected by checking the Syntax, you will find an "__r", as shown in the image below.
This issue can be fixed by migrating the field to a Parent Object or Standard Object instead of using a formula field.