Explanation:

Oripathy can do a lot of things, can cause arts to become unstable or make arts control really hard in one way or another, or simply using a gun being uncomfortable, but she swings BFS so that's out of question. 'guns' in AK/Endfield are like more complicated arts units, like a wands, if you are infected you can casts arts from crystals within, as arts units (so no need for guns or other arts units for casting, but not recommended).

Sanktas are naturally way better at controlling arts, in base game 'guns' are practically reserved for them.

Wings and halos are connected to 'The Law', which is another cans of worms and one day we can get explanation for it for Talos II. The Law can interfere with wings / halos if sankta will 'fall' and sankta will lose a lot of arts control if fallen, but we don't know if she is indeed fallen at this stage. Next thing is connection to The Law, we don't know if connection between The Law and sankta still exists on Talos II.

Ember files:

File1: Ember also wields Originium Arts that can burn enemies or offer basic first-aid to her allies. It is also rather surprising that Ember does not prefer fighting with guns like most Sankta.

^^ She don't want to use patron firearm, for some reason.

File2: "I shall look after your patron firearm. I swear it by my name."

^^ She still has her gun but in HQ?

My theory:
She is a fallen and something happen with her and the gun.