The assembly's presiding officer has asked BBC Wales to explain why its main television news programmes did not cover "one of the most significant votes in the history of the Senedd".
Elin Jones has written to BBC Wales director Rhodri Talfan Davies and director general Tony Hall.
BBC Wales said it would respond to Ms Jones' letter "in due course".
Plaid Cymru leader Adam Price said BBC Wales had failed in its statutory duty.
A similar vote in the Scottish Parliament the following day was given greater prominence, Ms Jones said.
Writing about the vote, which took place on 4 December, Ms Jones said: "This was one of the most significant votes in the history of the Senedd and the absence of any real reflection of this fact on that day on Wales Today and Newyddion 9 was disappointing.
"By contrast, BBC Scotland's output the following day was dominated by Scottish Parliament's vote on the Withdrawal Agreement."
The presiding officer asked BBC Wales to explain why a vote of such significance was not considered relevant.
She said she wanted to "formalise briefing sessions" with BBC's journalists and correspondent to update them on Brexit developments.
Plaid Cymru leader Adam Price said: "BBC Wales has a statutory duty to report relevant important political stories to people in Wales, but here is a clear example of a total failure to do so.
"BBC Wales chiefs should now take personal responsibility for ensuring that all the corporation's platforms give appropriate attention to stories from the assembly and treat the institution with the respect it deserves."
A BBC Wales spokesperson said: "We have received the letter from the presiding officer and will respond in due course.
"It is worth noting that extensive coverage was given to the vote across a number of BBC platforms."