The transition rule or a non-empty value can be worked around by inadvertance for Open List fields.
- take a tracker with an "Open List" field
- as administrator define a transition rule with the concerned field in "Field(s) that must not be empty "
- as a user, type some value in the field, but without selecting an existing nor creating the value
- perform the transition
After these steps, the user has succeded in executing the transition, but the required field still has no value.
Note that this same way (3rd step), same users often try and fail at editing the Open List field in a simple edition. So a fix that also impacts edition at the same time would be ideal. This situation is understandable in edition, but definitively a bug for rules evaluation wich should prevent the transition.