PyMC samplers failing on simple Gaussian example.

[janfb]

Describe the bug
We are using a simple Gaussian posterior example to test the MCMC methods. The new PyMC samplers are failing for this example, producing c2st around 0.85.

To Reproduce
Please add a minimal code example that reproduces the problem:

pytest tests/mcmc_test.py::test_c2st_pymc_sampler_on_Gaussian

Expected behavior
Test should pass.

Additional context
See discussion here: #1124