HI FC,
If you use the non-I2C analog port, the reading is the same with/without w5500 shield.
the w5500 is connected the a4/a5 with scl and sda.
for mega, the sda/scl are 20/21.
So i think the w5500 shorts the a4/a5 with scl and sda, then the pull up resistor impacts the readings. thanks.
oard I2C / TWI pins
Uno, Ethernet A4 (SDA), A5 (SCL)
Mega2560 20 (SDA), 21 (SCL)
Leonardo 2 (SDA), 3 (SCL)
Due 20 (SDA), 21 (SCL), SDA1, SCL1