This step is caused by a known 'feature' of the ADM3055 which is unrelated to the on-chip dc-dc power supply. The first bit in the arbitration phase will have this notch or step when the transceiver has been in the recessive state for longer than approx. 25ms. The level of the notch can vary based on the amount of time in the recessive state, but the time of the notch will remain the same.
The cause of the behavior is well understood and is consistently repeatable. While this behavior isn’t desired or ideal, it also has not caused any actual functional issues or problems in practice since the reduced VOD remains large enough for the other nodes to still recognize the dominant state.
This is unique to the ADM3055's transceiver die (and the other parts which use this same die - ADM3050E, ADM3056, ADM3057 and ADM3058).
When the transceiver has been in the recessive state for longer than approximately 25ms, it is possible for an internal protective circuit to be incorrectly triggered for a brief time causing a contention between the output driver and the protective circuit which results in the notch observed in the output CANH waveform.