When using the eyedropper tool in isolation mode on artwork, the color is taken from the actual artwork even though isolation mode displays it as faded. When doing the same on linked assets, even if they are Illustrator files, the isolation mode fade gets applied to the sampled color. This means that you have to back out of isolation mode and direct-select it to sample the asset, or otherwise have already created a swatch before entering isolation.

I think it would be nicer if Illustrator could treat the sampling of linked assets the same way it does for local artwork - sampling the actual colors instead of the colors after fading is applied.