Collider in RCT Subgroup Analysis

If the variable is pre-randomization and solely prognostic for the outcome, then adjust in the analysis. All that stratification aka subgrouping (or adding a product term in regression which is the equivalent of stratification) achieves is overfitting and nothing else useful can be gained from this procedure. This is a form of effect heterogeneity called risk magnification. To be an effect modifier, the variable must be associated with the intervention in a post-randomization sense as well as prognostic for the outcome, otherwise how would it modify the effect of the intervention on the outcome?