Title says it all. I've tried with multiple asset bundles of my own making and making of others. No asset bundle will stay in a player hand.
Can confirm I have the Hand flag turned on.
I've tried with many sizes of assetbundles (for testing possible collision issues) and none of them worked.

When the Hand flag is enabled, I do have the "Draw" option in the right-click menu - when I click it, the object moves to the hand location and hovers there until I interact with it (or hover over the object for a second) at which point gravity takes over.
The object never shows up in the on-screen hand - ever.