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When you run Pulumi up with an import - select details in the confirmation and you will see what doesn’t match

Thanks. Turns out it was pretty obvious in hindsight. What I didn't realise was that it had to be exactly the same to be imported. I was thinking that it could handle minor differences provided they didn't cause the resource to need replacing. The difference here is that pulumi was automatically adding a comment

"Managed by pulumi"

so I had to explicitly set the comment to

""

.