Can DC gain be smaller than the very low freq. AC gain in this amplifier?
I think you are in a rathole. It doesn't make any sense what you are seeing. It must be some type of measurement anomaly. I suggest you use two voltmeters to measure DC gain. First, check for voltemter offset by connecting them to the same voltage at the same time. Note the offset, if any.

Then, connect one voltmeter to the input of your filter, and another one to the output. Apply a stable DC voltage to the input. Measure the output. Correct for any offset if necessary.

Then calculate Vout/Vin using voltmeter data. Compare with gain at 1 Hz, etc.