Like everything, details matter.

The FO side is shrinking* to fund Captain upgrades. For 85 Captains to upgrades, 85 First Officer positions are removed, and these are removed unequally across the domiciles. This causes those on the bottom of shrinking domiciles to be displaced elsewhere. So from the perspective of a very junior person who gets displaced, there are still on the bottom, but now in a domicile where they have to commute, or have a worse commute. The next couple bids will see the FO side shrink even more, so they could be shuffled again, if their new base now shrinks. What fixes this is new hires.

*approximately the same amount of FOs will be bidding as ExTO First Officers return to replace upgrades, but that does nothing to help the relative seniority of someone on the bottom.

Vacation bidding seniority is already set with the September award.