The main issue is you new model has incompatible shapes with the original one regarding x, and you’re not resampling x.
Your error is similar to sampling a variable x of size 2 and then trying to use that trace in a new model where x is defined with size 3.
Partial observation creates two variables x_obs and x_unobs and in your second model x_unobs has a different shape than the one that’s found in the trace. I’m not sure what your goal was, as sample_posterior_predictive can’t do inference, it can only reuse draws from the trace or draw posterior predictive draws for a variable. In your case these would be prior draws since x does not depend on anything else