GuaranteeCheckServicers have the obligation. They're just not volunteering to honor it.
When Sunnova, SunPower, and PosiGen went through bankruptcy, the leases and PPAs they sold didn't just disappear — they got reassigned to new servicers: SunStrong Management, Sunrun, and Renewbrook. The production guarantees written into those original agreements transferred along with everything else, in the general case. What didn't transfer was any real proactive effort from the new servicers to check whether a shortfall exists and pay it out. Public complaint records describe requests being refused or ignored — the obligation is real, but nobody's running the math for the homeowner.
GuaranteeCheck runs that math. It's built as a self-help tool on purpose, not a collections service — the output is a letter you send yourself, in your own name, so there's never a question of anyone acting on your behalf without your say.