Spinda or Seadra Issues?

I noticed something odd recently with Spinda.  My opponent had a Horsea that had evolved into a stage 1 Seadra in their active position and also had a second Seadra on their bench. The Seadra in the active spot, had 1 energy, used the swim freely attack and successfully flipped a coin for heads.  This placed the effect, "If heads, your opponent's next turn, prevent all damage from and effects of attacks done to THIS Pokemon" onto my Spinda.  The next turn, the opponent retreated their Seadra and Spinda's damage succeeded on the new active Pokemon and 1 other on the bench but was prevented to not only the retreating Seadra but also the additional Sedra on their bench. Okay sure... so maybe that damage prevention is meant for "any: Seadra and I can live with that, but what really seemed off was all of the following turns. There were no stadiums, items, or trainers or other card abilities preventing any damage yet all future damage from my Spinda was prevented to both of the Seadra's for the next 3 or 4 rounds. It felt like a bug with the Seadra's effect that it wasn't lifted after 1 round but persisted for the rest of the match.