What conditions are most likely occurring if you feel you need a "Quality Assurance and...

A “Quality Assurance and Release Readiness Sprint” is not a Scrum concept, but a sign of poor engineering practices and weak adherence to the Scrum values and principles. It indicates that the Scrum Team is not delivering a potentially releasable product Increment every Sprint, which is one of the core commitments of Scrum. This could be due to a weak or incomplete Definition of Done, which allows technical debt to accumulate and compromises the quality and usability of the product. It could also be due to the Developers finding it nearly impossible to create a valuable, useful product Increment in a Sprint, which could be caused by various factors such as lack of skills, tools, collaboration, feedback, or empowerment. The Definition of Done is not being met every Sprint, which means that the Scrum Team is not fulfilling its accountability for the Increment and is not transparent about the state of the product.

References:

Scrum Guide 2020, section “Commitments”, “Definition of Done”, “The Sprint”, “The Increment”.

Advanced Knowledge of Scrum, section “Definition of Done”, “Technical Debt”.

Scrum Master Accountabilities, section “Accountability for Effectiveness”.