fixed_parameter_keys not correctly saved into the .h5 file
As the title says, something is going wrong here. I think it is related to the following line in run_sampler:
result.fixed_parameter_keys = [key for key in priors if isinstance(key, prior.DeltaFunction)]
isinstance is sometimes nasty and does not work as you'd expect all the time. Going through this, result.fixed_parameter_keys is still empty even if all but one parameters are fixed.