I think this is actually a common event, and I recall games where it's happened with Charm + other cards. (And maybe with 2 completely different non-Charm cards as well.)
I'm in my Buy stage, and putting my treasures out, and I come to Charm. (War Chest is still in my hand at this point.) If I play Charm, and then pick the coins-plus-buy or the get-matching-cards options . . . I cannot add more of my treasure into the pile. Any treasure still in my hand [e.g., War Chest, here] is "wasted." But nothing in the text of Charm suggests that this will happen.
I mean, if I choose the "get equal value cards" option, then I understand, since I have sorta forced the online game to the card-buying phase of Buy. But with the first option; all I have done is said to the game, "Add 2 coins to the treasure I've already put in, and add one more buy to my benefits." In our IRL games, people routinely have been playing treasures after playing Charm. Have we been actually breaking the rules all this time?
Is there something in the actually official rules that addresses/explains this? Or is this result merely due to how the online game was programmed to function?