If you summon a sword made out of fire and walk into anti-magic field, your firesword ceases to exist. But if you create an ordinary sword with magic, it doesn't dematerialise just because you walk into an anti-magic field, it's just a physically ordinary sword and as such not affected. Only things that can be dispelled are, well, dispelled.
So if you want to explain it you could just handwave it with whatever changes the netherese magic did to the tadpole was physical and not just enchantments, so it physical state remains the same even when in an anti-magic field.
Or whatever.
I love how things start fit together once you ask how instead of why.
