Treading carefully as I'm not permitted to use "engineering knowledge" or provide detailed insight. I'd limit the returned source value and monitor where it's being capped. Typically, the solver may fail if source terms are excessive, or very rapidly changing due to whatever is controlling the source. In this case it's more obvious that the high & low volume fractions fall off a plateau: just because the maths says so it doesn't mean it's physically sensible.
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