Dave Pelz did a study years ago with chipping. His reccomendation was always leave the flag in when chipping. The only exception was when the stick was leaning towards you. This putting test showed the same.
The USGA didn't realize when making this rule that there's different diameter flag sticks. The flag sticks with the reflective glass are larger than the cheaper non reflective glass sticks. Bryson Dechambeau has said there's a difference in COR with the two types of flag sticks also.
Hank Haney said the USGA is freaking out now that they realize that there is an advantage to leaving the flag stick in. Stay tuned.