You never "lose" xp you gained from xp items, regardless they are from TH or somewhere else in the game.
There are two kinds of xp items - lamps and stars. Lamps grant you xp immediately but stars give bonus xp to skill(s).
Your bonus xp will never get lost even if you the laziest kind of players who never play the game because there are other forms of lamps (e.g. Smouldering Lmaps) that automatically convert bonus xp to actual xp. There are also NPCs that come to the game sometimes to allow you to convert bonus xp to other forms of goodies, including but not limited to actual xp, useful items and cosmetics.
10-Feb-2019 16:30:16