Distortion in CAN signal
I've seen something similar before in a setup of mine - it turned out to be a measurement error.
Since CAN-Communication is differential you should use a differential probe to measure your system precisely (one probe to CAN-L and the other one to CAN-H)
By using a single-ended probe your reference changes and seemingly introduces errors in the waveform.
I wouldn't worry much about the shielding of the breakout-box; your impedance seems alright and your cable length is not very long.