Wrong ground reference producing wrong output on ADS1115
Unlike other devices in ADS111x series, ADS1115 includes source MUX that selects which of the input channels will be used as inputs for differential amplifier.

Since you are measuring shunt voltage referenced to the ground, you have to select GND as one of those inputs. See "9.3.1 Multiplexer" diagram in the datasheet. This is done by setting bits 14:12 in the Config Register to 0b100 (for A0 input).

Also, many ADC chips have "zero offset", which can be accounted for by calibration. I don't know if that is the case with this particular chip.