If you take a UDF and place it Right of another field
If you then place one more item above or below the item you place right of you will notice it does not give you a label
This behavior is an error but because a fix for it would cause lower performance, we have decided to leave the bug be.
The workaround for the behavior is to go to the Item Placement Tool configuration and remove the value in the label relation field of the configured line
Once removed the result is as expected