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The Summarize tool returns NULL when performing a Mode operation. This doesn't seem to be documented anywhere in Alteryx documentation nor the community. Please fix this behaviour.
I just tried using Mode on the Summarize one tool example in Designer and it seems to work with and without AMP for 2022.1 and our Dev build for the 2022.3 Release.
Unless there is maybe something specific to your data. Could you see if you can reproduce using the Summarize one tool example and changing the first Summarize tool to use the Mode action?