Answer
This has been tied to reconfiguring an existing task's type in place — for example, starting a task as an IFS Process Enrichment task and then switching it to an IFS API task — leaving behind internal configuration fields from the original task type that the new type doesn't expect. Deleting the problematic task entirely and creating a brand-new task of the correct type from scratch (rather than converting the existing one) has resolved this exact class of validation error.