I've worked with git using the mega-merge approach, and one thing I found is that git-absorb won't merge commits into anything that precedes a merge. It works fine for absorbing changes into earlier commits on a feature branch, but not from the WIP branch back into the multiple feature branches that are the parents of the mega-merge. jj handles this with no problems.
From this comment on the git-absorb issue tracker I wouldn't expect it to be fixed soon either: https://github.com/tummychow/git-absorb/issues/134#issuecomm...