In some integrations I’ve seen it output the structured output multiple times, with or without differing data, as a way to (undesirably) provide JSONL instead of using a simple array as intended. So, the main issue here appears to be that the model is allowed to continue outputting after it has completed its object, and being constrained to valid JSON symbols isn’t enough to prevent these issues.
As a workaround, the Chat Completions API has parameters for presence and frequency penalties. Using either may encourage the model to terminate its output when it’s finished.