What I am trying to say is that, the current behavior never intended to modify the random method of a distribution, as the random method is always assuming to produce random sample that satisfy the constrain. Thus user should not expect transform to be apply on sample_posterior_predictive and sample_prior_predictive.
It is however a valid feature request to have transform apply on a distribution so that it does the correct forward and inverse on both samples and log_prob computation, for that we could introduce a new transform distribution class like the one in TFP.