The problem risk assessment is a simplified risk assessment with lower documentation requirements than an IT system assessment. Use it for problems that are not bound to a specific system or to the organisational risk assessment. It is tailored for cases such as the Log4j vulnerability, where you quickly need an overview of the situation, map out the implications and implement countermeasures.
Risks from a problem risk assessment are listed with risks from your other risk assessments. You can filter on the risk assessment type in the risk assessment list and in the risk registry.
Not all risk assessments fit the IT system approach or belong in the strategic organisational risk assessment. Sometimes you just need to assess a specific problem and decide how to deal with it. The problem risk assessment gives quick access to the bowtie analysis with fewer documentation requirements.
Use it, for example, when a new problem arises that affects large parts of the organisation, and you need to assess it quickly and delegate countermeasures.