In 2006, 2014, and 2018 this wasn’t an issue in the FOMC minutes because at the January meeting, the outgoing chair was already preparing to leave as governor and the incoming chair had been on the cusp of (days away from) Senate confirmation. The new chair had also already been sworn in by the time the January meeting minutes were published.

So this is one part quirk of the transition coming roughly five months (instead of five days) after the January meeting officer elections, but also a good reminder of FOMC governance — the default position is that Powell is chair of the FOMC until he’s not.

In 2018, Powell had been confirmed as chair on 1/23/18, for a term beginning in Feb, so at the FOMC meeting one week later, they had two votes, one to elect Yellen as chair through the end of that week (to 2/2/18) and a second to elect Powell as chair of the FOMC effective 2/3/18.

Correction: In the case of Yellen (2014), and Powell (2018), the Senate had already confirmed them before the January meeting. And in the case of Bernanke (2006), he would be confirmed later in the day of the January FOMC meeting. So the committee in the case of Yellen and Powell elected them as chair following the conclusion of that week. And in the case of Bernanke, they made clear they would quickly hold a "notation vote" to make the incoming chair of the board also the chair of the FOMC.