U26-001 readings drop to zero

I have a U26-001 logger recording DO in a river and the DO levels drop to 0.0 mg/L. We have completed the laboratory calibration prior to deploying the logger and have completed 2 field audits (not during 0.0 mg/L readings) that have verified the DO levels to be accurate. I'm concerned because 1) these are winter DO levels and should not be zero and 2) the levels change drastically from normal readings to zero. Has anyone else had these issues? http://prnt.sc/mnseht
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