SP and RP eligibility exist solely for search purposes.
Jalen Beeks is coming out of the bullpen - he is not starting, he does not get a GS. As a result, he has to be used in a RP slot.
The reasoning for this rule is that it prevents using pitchers who have both SP and RP eligibility but pitch primarily out of the bullpen from being used in SP slots on days you don’t have 5 starters going, etc.