I have an app that calls a flow that calls a stored procedure. I have an output parameter in the stored procedure named @ErrorMessage defined as VARCHAR(4000). When I call the stored procedure from Flow, I accidentally put OK into the parameter value when called. What happened next I did not expect.
While testing, I triggered the Flow that should have returned an error message. Checking the run history showed a proper error message returned from the stored procedure, but PowerApps only go tthe first two characters to display to the user. Coincidentally the lenght of the value "OK". Removing OK from the parameter value passed to SQL Server, now returns the full error message.
Something between PowerApps and Flow was truncating the poutput parameter. This should be fixed.