Op-amp voltage buffer attentuates the input
Per problem description, all indications are that the input impedance of the tested circuit is fairly low, under 100 kOhm or less. Which isn't expected, since the input leakages and offsets for the LPV821 are of the order of 10 pA, which should translate into about 100 GOhm effective input impedance.

However, the LPV821 is a very delicate precision "chopper" amplifier, nanopower:

 

The datasheet is full of notes on how to protect pins from overvoltage or overcurrent. Having accidentally only 4 V can damage the IC. Given questionable design of power rails in this question, I would say that this test sample is permanently damaged.