Peculiar i2c issue

[nihalpasham]

I've hit a wall trying to connect my nRF52840-mdk with a specific i2c slave (that sits in a port-expansion board). If you need more context, here's a link to the problem - https://github.com/nihalpasham/testnRFi2c

Although, it does work with other i2c slaves (such as my mpu6050 sensor), I'm guessing this has something to do with the nRF-mdk's internal pull-up resistors. I couldn't find any documentation on the resistor values? Could someone shed some light on this?