Just wanting to double check I''m not going mad. The datasheet shows that a 5v Vdd, a 140mV ref and a signal going up and below from 0 to 500mV is well within input-mode offset and overdrive for the slow frequency, but the output is always driven to a 700mV value (15k pull up to Vdd). I purchased these from lcsc.com and while they're usually good at ensuring brand name parts are genuine I do wonder if these aren't.
Tried 5 pieces now with the same outcome.