5 minutes after I shielded I was attacked and lost points,
i expect to lose points occasionally after shielding but not 5 minutes!!
You will only lose points while shielded if someone was in a battle with your team before you shielded. They get points as usual. And because battles can resolve at any time, this could be 5 minutes or even 5 hours after you shielded.
As pointed out by the others, there are cases in which client-server latency will delay these message prompts when a player defeats you before shielding, but you receive this prompt after shielding. I’ve personally experienced a case where I hadn’t received this prompt for an hour, but this was mostly because I closed the game directly after shielding without waiting or triggering any update prompts on my end. Sorry for any confusion this might have caused, but this appears to be a known issue caused by experiencing client-server latency.
You can verify this yourself by viewing the leaderboard after your shield. It may show a lower value than you saw just seconds before, as it polls the server for the most up-to-date scores possible. You might receive the message for the hit(s) that lowered your score several minutes later, but the battle was already finished.