This is Auction Overlap
If you have multiple ad setsAn ad set is a Facebook ads grouping where settings like targeting, scheduling, optimization, and placement are determined. MoreClick word to read full description that attempt to enter the same auctionFacebook uses an ad auction to determine the best ad to show to a person at a given point in time. The winner of the auction is the ad with the highest total value, based on bid, estimated action rates, and ad quality. MoreClick word to read full description, the algorithm will choose the ad with the highest total value. This is Auction Overlap. The other ads won’t be considered.
This is most likely to happen if you have multiple ad sets targeting the same people. Auction OverlapAuction Overlap can happen when you have two ad sets running at the same time, targeting similar audiences. When their ads are about to enter the auction, Meta first chooses the ad with the highest total value. That ad will be the one that enters into the auction. The other won’t be considered. Meta does this to prevent you from bidding against yourself. When there’s too much Auction Overlap, it can result in higher costs or under delivery. MoreClick word to read full description is meant to prevent you from competing against yourself in the auction.