There is an incoherence in the context management in proteus.
Until r 4a754a2b76c3, the context was always taken from the _config attribute of the Model. So now, we have two place where the context is taken and they may be different.
I think the context management should be more coherent and an instance should always use the same context during its all life. This include the related record instantiated by xxx2xxx fields.