Collider in RCT Subgroup Analysis

Yup, this remarkably came out at the exact same time we described the same thing using similar DAGs (a major difference is that we adapted them towards selection diagrams and described the do-calculus considerations). See also here. The potential collider bias you point out is that @Stephen is eloquently refers to when cautioning that in subgroup analyses we can assign individuals their treatment levels but not their covariates.