I have this weird problem with cse_global and the fluctuating LB. It works fine with SymPy 1.2 and produces wrong temperatures with SymPy 1.1.1. I have bisected it down to this change: https://github.com/sympy/sympy/pull/13221. My patch disables cse_global if it's enabled and displays a warning on the incriminating SymPy version. I used Mathematica to plug the subexpressions back into the main expressions and get the same thing with both SymPy versions, so I have no idea what the actual problem is. Thus I can't rule out that it only occurs with the fluctuating LB. Until we know for sure, it seems safest to just disable cse_global on that old SymPy version entirely.