Summary

Custom Field Wiki Style Renderer Has User Mention Button Which Custom Field Doesn't Support Mention In The First Place

Step to reproduce:

• Create a custom field with Text Field (multi-line)
• Assign to any screen
• Navigate to Field Configuration page (or press "gg" and search for Field Configuration)
• Click on Configure button on any field configuration that contained your custom field.
• Navigate to your custom field and click on "Renderers" button.
• Choose Wiki Style Renderer
• The custom field will look like the below image:

In conclusion, User Mention button does not belong in that custom field since custom field does not support mention. There is a feature request for this case.