It's not entirely clear when a creature loses defender
My opponent had Prismari Pledgemage and played an instant or sorcery spell so that his Prismari Pledgemage lost defender. The problem is that it wasn't clear at all that he lost defender, the only indication that that was the case was that the defender icon was crossed out.
Normally when creatures lose abilities, the ability is grayed out and hovering or right clicking says "abilities removed", but not in this case (if I remember correctly that used to be the case even in this case, so this seems to be a recent change).
Prismari Pledgemage could attack as it should, the problem is just that it's not clear from the UI that he can attack.
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[Deleted User] commented
Prismari Pledgemage doesn't lose defender, it's trigger simply allows it to ignore that ability for the purpose of declaring attackers. It still has the defender ability for effects that would care about such things.