`git show $HASH` -- does that show the abc delta? If not, then you're seeing something either on another leg of a merge, or else you're misreading the log -p output. I'd guess.

And yet `git log abc` doesn't show HASH -- hrm, is HASH in the history of your current tree?

You have an interesting situation, and without poking at it myself I'd find it hard to make any other suggestions.