Skill readiness looks different to a parent than a coach - especially to parents that may not have many years experience in the sport.
@Lobster gave a good example with readiness for skills that progress out of a kip, even if everyone has a kip and may appear 'equal' to a parent. Do all the kids on your DD's team compete the same routine and skills? If they are competing different skills you may be making an apples to oranges comparison. Also, if the kids are moving up to platinum next year, depending on the gym and what is competed in platinum, this could mean a big jump in skills and it would not be surprising that some in the group may not be ready to advance. (gold to platinum has more variability than say moving from bronze to silver).
My DD was not the 'favorite' with one of her event coaches. At times this caused her much grief and confusion. The favoritism however looked much different than what you describe. It was more personal such as the coach giving high fives to another teammate and no acknowledgement to my DD. It was the coach never commenting when the favorite slacked off in conditioning but doing with opposite with my DD. It was the coach rolling her eyes at my DD. My DD in her 10+ year career in club gymnastics did experience the uptraining gatekeeping you describe but that in itself was not favoritism....it was just that she wasn't ready (and other times she was on the other end and got to uptrain while others didn't).
You have received consistent feedback here that the lack of uptraining is not necessarily an indicator of favoritism and is most likely an indication that something is deficient in her readiness. The only way to know for sure is to ask the coach.