First of all there is the test that Bonifatius did. The piece of cloth he tried to cut with his transformed schtappe completely nullified the attack, rather than just refusing to be cut. WIth his monstrous strength, the stand the cloth had been lying on, should by all means have been broken, but wasn't harmed... which strongly indicates that mana based attacks and items should not be able to even move silver cloth.

Second, we've seen Detlinde being hit by explosions from Ferdinand's magic tools and not only not suffering the direct effect, but also not being thrown across the entire room into the next wall, which again quite strongly suggests that magical effects do not work on silver equipment.

Third is the ease with which the Lanzenavians are able to carve through the Ahrensbach knights. Granted, it is not impossible that they used exclusively poison and the three that attacked Eckhart and Angelica just so happened to have run out of poison and thus had to fall back on their weapons to attack them, but that seems quite unlikely.

And finally, even the guy who threw the knife was surprised it didn't work. And while I can buy Rozemyne's assumption that the attacker wasn't aware that magic charms of this specific type existed, I doubt that with the proliferation of magic tools in Yurgenschmidt, especially rich greater duchies like Ahrensbach, he'd be completely oblivious about defensive magic tools existing in general. And yet he seemed quite sure that the silver weapons would be able to overcome whatever magical defenses the Yurgenschmidt nobles could throw at them.

None of the points is definite proof, but they do make a pretty strong case that silver equipment should not be affected by magical effects.

Now if Angelica had deflected it by hitting the flying knife out of the way with her hand, I would have no issue. She's fast and precise enough to hit the flat side and thus get away without injury, even if it can go right through her armour. Likewise knocking it aside with Stenluke should be fine, since a mana blade has a physical core and is not just pure magic given form. But the charm activating seems a bit odd to me. It's nothing that breaks the story or anything, but I would really like to have some answers here. Maybe one of the Fanbooks will have those in the Q&A section.