There is difference between UI and Workflow behavior and the way how custom fields show filtered results:
- Custom field in UI shows results according to Filtering settings.
- The same field in Workflow shows all results and is not filtered.
There may be situations where two or more custom fields are linked between each other and filtered on one form. As a result, invalid combinations can be chosen in Workflow.
The current functionality is by design. In this particular case, Workflow can be compared to scripted program. If exact values are set in a code and ran, the user will face some invalid combinations - because it is set before the actual process is evaluated and cannot react to user interaction.