We have observed the below behaviour when you upgrade your Rasa version from 2.3 or 2.4 to 2.5 and the use same the tracker store.
The rasa server struggles to respond, due the version difference the policies takes longer time eventually resulting in timeout.
I am not sure if anyone has tested this scenario. We faced this issue when we upgraded our production rasa chabot .
I am not sure if there are any guideline to avoid such scenario