I am using the i2c_t3 library for teensy (https://github.com/nox771/i2c_t3) and have modified the basic slave example provided. The master device addresses the slave, and requests two bytes of data from the teensy slave device. After testing it with a master device (a FPGA), I was unable to get the teensy to send 2 bytes of data to the master device. The teensy only sends back one byte of data, skipping the first byte. Below is an oscilloscope screenshot of the communication between the two devices.
Click image for larger version. 

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I have also attached the slave code to this post.

- Sam