No warning is a good thing I guess 
- Have you tried with a more regularizing prior? Like a Gamma, or else
- Maybe it’s because you just have one observation? Thus, even if your prior is super wide, it can’t be overwhelmed by just one data point. If I’m not mistaken, the max like estimate of your prior is 150, so getting to 103 after sampling means it was moved by the data, but not to the point of completely coinciding with it. This is only conjecture of course.