A few weeks ago it seems the resists on shields have been changed and no longer apply passively, so on any kind of fire situation, for example, it wouldn't grant any extra fire resist. (or rather, it wouldn't reduce fire's duration or power at all)

The resists however help the shield survive longer against monster attacks that would inflict that specific status effect. (for example, a Grey Owlite would resist Sparky's gun bullets and Red Rover's flamethrowers pretty well, however Sick or Slush puppies would be able to break the shield much more easily, since it does not resist poison or freeze)

But I'm not really sure if this is exactly how it works, because of one specific thing: Vanaduke's Fire Snakes.

It's pretty simple. I use a Vog Cub Cap with Maximum Fire Resist UV, and a normal Vog Cub Coat, and a Grey Owlite. I would guess because of the cap, I am immune to the snakes - My other friends and guildies who used Vog Cub aswell, without any UVs but with a Volcanic Plate instead of Grey Owlite would get set on fire by the snakes.

But I once ran Vanaduke without the coat, using Ancient Plate Mail instead, which does not offer fire resist - Overall, I ended with less fire resist than a normal Vog Cub set without variants would offer, (...kind of weird? If stacked with the armor's own fire resist, it seems the effect of the UV decreases a little, but if not stacked, it offers as much as a 5* armor) yet... I was still immune to the fire snakes.

But that is really the single only thing that I have ever noticed, for passive resists. In general, it's safe to say that resists are not passive and only make a shield defend X attacks better.

And as a note, shielding during the status effect does not do anything at all. Except for shock, which will block the tick's damage. For fire, it'll deal damage regardless.