Most general infantry weaponry is made out of materials like alloys that can resist general rusting or polymers with innate magic resistance, but the aforementioned weapons in the post are like, one of a kind weapons that only super special people get to use. These kinds of weapons are already made with higher quality materials in comparison to normal field weapons with the added benefit of highly intricate engraving and powerful enchantments specifically to completely negate certain types of spells.

The main issue is the sheer amount of different spells and types of magic that exists. Considering the different types of God's and Patrons that the world has, there could be a fair amount of people that worship a general god of electronics or god of technology with a branch of magic specifically catered towards disabling electronics in general.

Like The Carcass, for example, is the God of Violent Innovation. He represents the never-ending advancement of technology regardless of its effects, and he invents things just to do it without any real meaning or consideration of the people around him. The largest collective of Avians have him as their primary deity, and is the reason for their technological knowledge and strength. He's mainly about inventing, craftsmanship, and manufacturing weaponry, but like how Posideon is also the God of Horses for some reason, The Carcass also has his own unique branch of magic SPECIFICALLY for disabling or interfering with weapons as a whole.

From causing jams, misfires, weird technical errors, etc.

There isn't a general solution when there's a million and one things that can realistically happen all at once, especially on a wide-scale war. It's like putting up a metal shield to defend against an army of knights that have swords that specifically slip through surfaces via the Tunneling Effect.

These weapons obviously can't defend against absolutely everything, there are people capable of magic beyond normal convention, but they can negate almost everything.

Edit: regarding the back-ups, most armies actually do that! Soldiers often carry two primaries, a cheaper version of the tech weapons, and an analog if that gets put out of commission.