Did you write the MPS file using Pyomo's built-in write functionality? An MPS file generated by Pyomo does not necessarily capture the exact model as stored internally by Gurobi, and in some cases may not be compatible with Gurobi.

To write out the MPS file using Gurobi's own file-writer, you can set the $$\texttt{GURO_PAR_DUMP}$$ parameter to 1. E.g., in Pyomo:

opt.options['GURO_PAR_DUMP'] = 1