The Developers keep their own list of technical debt and infrastructure work in a separate tool, and pull from it whenever they have spare capacity, so that it never has to compete with features in the Product Backlog. The Product Owner is aware and content with the arrangement. What is the problem?

Professional Scrum Master I, objective 1. Understanding and applying the Scrum framework hard

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The options

Correct The Product Backlog is the single source of work undertaken by the Scrum Team, so a second list removes transparency and makes the Product Owner's ordering a partial picture

Correct. Everyone downstream — the Product Owner ordering, the stakeholders at the Sprint Review, anyone forecasting — is reasoning about a product whose real workload is larger than the artifact says.

Not correct Nothing — technical work is the Developers' business and does not belong in a product backlog

Wrong, and it confuses two questions. How work is done is the Developers' business. Whether work is undertaken at all is a product decision, and this work consumes the same Sprint as everything else.

Not correct The only problem is that the Product Owner cannot size the hidden items

Wrong. Sizing would still be the Developers' job even if every item were visible. The failure is that an entire stream of work is invisible at the moment the product's priorities are set.

Not correct The Developers should stop doing technical work unless a stakeholder asks for it

Wrong, and it is the opposite mistake. Technical work is real product work; the fix is to put it in the Product Backlog where it can be ordered against everything else, not to stop doing it.

Why

The Product Backlog is an emergent, ordered list of what is needed to improve the product, and it is the single source of work the Scrum Team undertakes — which makes any second list a transparency problem rather than a tidiness one. The usual objection, that technical work will always lose to features, is a real risk and is answered by the Product Owner understanding the trade-off, not by hiding the work from them.

Where this comes from

Cited
Scrum Guide section Product Backlog
What it says
It is the single source of work undertaken by the Scrum Team.

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