I am having this same issue. Saving the flow and then recreating with the template and deleting the old one works but is a horrible solution to what seems like a clear bug.
I shouldn't have to recreate the whole flow when I want to add a powerapps parameter to an existing flow.
Instead of creating an entirely new app (templated or otherwise), try this:
1. Click on the View tab at the top
2. Select Data Sources
3. Find your flow in the list, click the "..." and "Remove" it
4. Click on the Actions tab at the top
5. Select Flows
6. Find your original/updated flow, and add it back
The UI, tab, option locations may change in the future, but ultimately remove the Flow reference from the app and add it back. Flows don't appear to auto-update, which is probably a good thing, although there should be an easier way to "Upgrade" your flow to the latest version without having to jump through these hoops.
I had the same issue, i tried to save as the flow with a new name, removing from Powerapps and adding back, without resolving the issue.
So i fixed my issue editing the flow by recreating the step where i asked for parameters to PowerApps (the step was a sql stored procedure). After that i went back to PowerApps and when i add the flow i could see the new parameters.
I hope this could help some one
I want to add that this method (removing the flow from the PowerApps "View -> Data Sources" menu and adding it back in the "Action -> Flows" menu) works but has error under some circumstances. For instance, our flows are developed while logged in under a service account that differs from the accounts we use to develop PowerApps. It was causing us a "failed to register" error when atttempting to add the flow under the Actions tab.
To resolve, had to log into PowerApps using the same service account that was used to create the flow.
Also be sure to copy the text of your flow "Run()" command to paste back in to the new "Run()" text that will be inserted (otherwise your existing command text might get overwritten).
I also tried to use the PowerApps "Action -> Flows" menu to select the "Edit" option to edit the flow but again if logged in as a different user it will force you to enter credentials to rebuild all the connections. If you log into PowerApps as that other Flow user account you do not have to enter new credentials.
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