PowerApps Custom Form, Edit mode, "The requested operation is invalid"

[Anonymous]

Hi folks.

I've made a custom form for a SharePoint List; it works fine when I create a new item, but it generates "The requested operation is invalid" error each time I try and save a Form from edit mode (whether or not any changes are made).

The App has separate screens and forms for Create, Edit and View, so I'm using the SharepointIntegration OnNew, OnEdit, OnView, OnSave and OnCancel values as directed here:

https://powerapps.microsoft.com/en-us/blog/separate-custom-forms/

To which I've addedd some code to Refresh the data source before launching the forms, just in case that has an impact, as suggested here:
Custom form intermittently showing error 

As far as I can tell there's no issue with the permissions of the account I'm attempting to use to edit the List items: I've switched back to the default data entry form and that was able to edit the data of existing items.

I've turned off Explicit Column Selection just in case the SubmitForm() somehow doesn't know which List item to update if it hasn't loaded every column value? This is basically superstition on my part.

I thought this was working yesterday but my memory is clinically poor and I didn't document explicitly that it was, so I'm not certain. I have added a PowerAutomate process to a button on the form since yesterday, but I have no idea how connecting in PowerAutomate would prevent updating of records.

There's only one required value in the list, but that's provided when the user creates an Item and isn't changeable in Editing mode, so I can't see how that could have an impact.

I'm not using any Patch functions.

Any ideas about other factors that would make SubmitForm() an invalid operation from editing mode?