As per the following forum entry if the definition of a Flow that is being used in a PowerApp is changed the PowerApp does not recognize the change until the Flow is disconnected and reconnected to the PowerApp.
https://powerusers.microsoft.com/t5/Using-Flows/changed-flow-in-powerapps/m-p/156590#M4113
The error that results from this scenario is not easy to diagnose so we should make improvements to the user experience in one of the following ways.
1. Automatically reconnect Flows with related PowerApps when the Flow is altered.
2. Display a warning message when Flows are altered that instructs the user on what PowerApps need to be serviced.