There are two different Provider domain records in the OMOP concept table. The record with concept_id = 33 has a corresponding record in the OMOP domain table and, interestingly, has domain_name = 'Provider Specialty'. This is the domain that has constituent concepts.
The other Provider domain record has concept_id = 55 and has been updated with invalid_reason = 'D' and valid_end_date = 2019-10-02. This second Provider domain concept record does not have a corresponding record in the OMOP domain table.
I speculate that this second Provider domain, and the other “empty” domains on my list, were created for use in columns domain_concept_id_1 and domain_concept_id_2 in the OMOP fact_relationship table. (See this post: Question about Fact Relationship - #2 by rookie_crewkie)
In other words, Provider with concept_id = 55 refers to the OMOP provider table, whereas Provider with concept_id = 33 is Provider Specialty. It seems plausible that someone thought there was a duplicate and logically deleted the one with no constituent concepts.
To my knowledge, nothing of consequence hinges upon how I populate the concept.domain_id column for my custom source vocabularies. Therefore, I’ll just pick one of the existing domain_ids and be done with it.