I have had this problem for months and have no idea how to resolve it. It is doing this with both Adobe Acrobat Pro X and XI.
I will hightlight the text in a bookmark with the intent to edit it, or select the text in a comment or a pop-up note, etc. so that I may copy it to the clipboard.
But before I can even either copy it, or start typing in the text field to edit it, my selection of the item drops. My highlight selection only lasts about one second or so.
This is very irritating.