I have 17 videos from the IBF Convention last year on YouTube. The first video talks about second day weigh-ins which only the IBF mandates. Specific examples stated were David Lemieux vs. Curtis Stevens and Gennady Golovkin vs. Daniel Jacobs.
Lemieux did not participate in the IBF's second day weigh-in, so he was not eligible to win the IBF middleweight title. On the day of the fight, the commission weighed Lemieux and he was 181 pounds. Stevens was knocked out.
Jacobs did not show up for the second day weigh-in. Jacobs' weight on the day of the fight was apparently unknown, but it is suspected that he weighed more than 10 pounds above the middleweight limit of 160 pounds. I previously stated on this forum that Jacobs weighed about 180, but I was mistaken because the IBF was referring to Lemieux.