Add two (even one) custom field of list type to the workpackage type. When you try to select one, the other one is autmatically reset. This happen even if you have only one custom field, just select any other combo box (like assignee or priority) just after you changed the custom field.

It seems the GUI erroneusly “reset” the last selected combo box to some default value when you modify a different combo box. This has been verified with Firefox. Begin of the insertion

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