My Pikachu ex terra in miraidon died with full hp

I was playing my miraidon ex deck against raging bolt in tcg live. We both had one prize card left and my pikachu should rescued me by winning some time, but it died after it was attacked by raging bolt ex, with full hp, and the opponent won. I saw the pikachu card going to the discard pile and lost the match. Is its ability not supposed to leave pikachu with 10hp in the active spot? I wanted to set up magneton's ability together with raichou on the bench and attack the opponent in the following turn.... -_-

Can someone explain me Pikachu's ability, please.