Going through the portal after defeating the ender dragon results in all items being deleted, apart from the first 27 items and the hotbar. (and armour, probably. didn't check)

Confirmed this does happen and it is seriously bad. I'm working on fixing it now

EDIT: Turns out I made a very big derp in the inventory transfer method

Why dint i see this earlier? f**k!
guess i lost over 480 lots of items

Yea sorry about that. I'm trying to fix this (4 failed dev builds with it still happening) but Forge isn't firing events at the right time to catch the items before they're erased. For the record this applies to every dimension teleportation (I'll change the title) not just the End.

PS: Try to not to use profanity in the issue tracker as frustrating as it is.

I am happily going back and forth between the nether and the overworld. Going to the end works just as fine, as well, only coming back is causing this issue.

Using mystcraft linking books has been safe so far, as well. (I have not tried custom ages, though. I installed mystcraft because of this problem.)

Odd, I have the issue even when going back and forth from the nether. Must have been something I touched while trying to fix the original issue.

EDIT: Well... apparently in creative mode the dimension transfers fail miserably -_- in survival it's just fine (except returning from the End of course).

Well this just gets even more interesting. Turns out it wasn't creative mode specifically doing it. ANY dimension changing after completing the end results in erased items. You'll just have to re-log to fix it after you leave the End

Well I've fixed it for survival mode but Creative is still very broken

EDIT: It's finally fixed! Update coming soon. Just need to clean up the ton of debugging lines I made