I just burned 40 levels and eight golden apples on a book of absorption that failed to work as advertised. Right after I used it, saw the heart appear between my armor and health bars, I teleported home (ender pearl). The heart of absorption disappeared. Maybe a second or two between these actions. Spoke with leo389 on the server, and am following suggestion that I post on forums.
Were you in a non overworld dimension when you used it? Natural absorption does not seem to work correctly with dimensions. (I have 11 extra hearts and they will not recharge in the nether/end/deep dark/mining world.)
I was farming pigmen for the last few levels of XP I needed. I used the book in the nether, saw the heart appear, then I threw down a pearl. When my client loaded my base, I saw that the heart of absorption was no longer there.
Go back into the nether and see if the heart appears, and from now on only use the books in the overworld(They will recharge in the overworld and stay when you go elsewhere, but not recharge until you return to the overworld). Probably need to bring the issue up with the mod dev.
So the problem is that the absorption hearts go away when you change dimensions. And you can't get them back until you relog. So the workaround is to visit the dimension you want your absorption hearts in, and relog. I'm going to make a bug report on the mod thread on the minecraft forums.
