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for posterity, I figured out that this wasn't a pulumi issue -- someone was doing something tricky with monkey patching with our pulumi codebase that had unintended consequences

Just pass the

id

of the instance to the group:

instance_group = gcp.compute.InstanceGroup(instances=[instance.id], ...other args)

See the example named

Example Usage - With instances and Named ports

in the docs: https://www.pulumi.com/registry/packages/gcp/api-docs/compute/instancegroup/