ResearchStudy.currentState 0..* CodeableConcept has a binding to a value set for study progress status values.

ResearchStudy.statusDate 0..* is a backbone element enabling date data (Period datatype) associated with an activity (CodeableConcept datatype) with a binding to a value set containing mostly study record status values.

This model confuses study progress vs. study record focus with dateless vs. dated status data.  An overall simpler approach to modeling is suggested with:

ResearchStudy.studyStatus 0..* BackboneElement containing 4 elements:

focus 0..1 code (choices of: study-record | study-progress)

activity 1..1 CodeableConcept (with Extensible binding to a value set including all prior value sets for the 2 elements noted above)

actual 0..1 boolean

period 0..1 Period

Relaxing the requirement for period allows use of this structure instead of a separate currentState element to enable classification without a date.