I2C multiple masters, one master has internal pull up, do I need external pull up?
I2C pullups are a system level consideration. You only need one pair of pullups per segment, but they should be sized with consideration for your bus load capacitance to ensure your digital signaling is sufficiently digital and your edge rates are within specifications. I always start around 2.2kOhms and adjust as necessary. 12 kOhm sounds way to high for my taste.  In general an oscilloscope is handy when it comes to evaluation of I2C signal in integrity.