There appears to be a bug, that I've read on various forums that other people are experiencing as well, where the dev person creates a new flow for executing a stored procedure, and they get this "extra" field/parameter called pInitId.
The problem is, there is no "X" to remove that parameter field, yet if you try to add a value to it, it tells you that the parameter is "no longer in the stored proc". It never was there!
I have since added a param with the same name, with a default value and hardcoded the value in the below screenshot, this works, but clearly should be how it is.