I have noticed repeatedly that some keep cards, especially Horned Plates (but others as well), when triggered sometimes are returned to the hand and sometimes are re-attached. Not sure if this is a bug but I cannot find a reason as no discard attacks were played when this has happened.
Notice that those cards have two description box. The Keep part only take effect when description in its box triggers. For example, if Horned Plates adds damage, it will stay in your hand since the second box said "Frenzy 3. Keep." but if it reduces damage, it will be discarded since the first box only said "Armor 3." without the Keep part.
As VNSAramaki said, Horned Plates can have two effects: If reacting as armor it is Armor 3. Notice this does not have Keep as a keyword. Therefore, if the die is 4 or more it triggers, reduces damage by 3 and is then discarded. If the die is 3 or less it does not reduce damage but returns to your hand. If boosting an attack it contributes Frenzy 3. This does have the Keep keyword. Therefore, if you play a melee attack it will do 3 more points of damage due to Horned Plates then Horned Plates will return to your hand. Hope that answers your question.
