IndependentMultiVariateDistribution and serializing models

When using IndependentMultivariateDistribution for the roi priors, I am getting and error when trying to save the model usingthe serialisation rather than pickle. The save as .binpb works fine when I use the same distribution type (i.e. LogNormal) for all priors. I can save it as a pickle without any issue. (These screen shots are from a modified Colab for Meridian Getting started). Am I building the priors ok as the model builds ok and using the priors.

The report names no source file or test; begin by reproducing the failure in the modified Meridian Getting Started Colab with mixed IndependentMultivariateDistribution priors. Compare serialization to .binpb with the working same-distribution case and pickle case. Done means the mixed-prior model saves successfully through the documented serialization path.

Written by the indexing model from the issue text.