It’s the perfect color now for damp arid ground...
A dry brush will help get it back toward looking arid, but probably not quite to how the unsprayed base looks. (Just an aside, you can also use a light dry brush on the uniform of very pale buff or sand that has a yellow tint to help it look ‘dry’ after using the wash for the shadows. Washes sometimes make figs look ‘damp’. Basically, the dry brush brings up the edges and folds like it is getting hit with sunlight. I use that technique on 6mm vehicles and it helps them ‘pop’ visually, especially ones with complex camo patterns like WW2 German tanks.)
It’s a challenge getting natural dirt to look dry after it has been sealed - I can’t think of any sealant that won’t darkened the base. My wife thinks I’m crazy going through the steps I do to make actual dirt look “right” on the bases.
Is the grit on the unsealed base glued down hard enough that you could get away without sealing the bases? They’ll shed dust a bit, but might hold up well enough. Some of my collection doesn’t have sealant on the bases and they’ve done alright.
Alternatively, and this might be a long shot, sprinkling a bit of fresh dirt on top of the base immediately after the sealant goes on may bring back the dry effect. My guess is that the material is darker after sealing because the clear varnish soaks through from the top and when it dries interacts with light similarly to a layer of water. By adding dust on top, hopefully the new layer will stick to the varnish, but won’t be soaked through, so it will keep the right reflectiveness. If you try that, you’ll have to be really careful not to get dust on the figure! On the other hand, maybe a bit of dust part way up the boots and pant legs would look authentic...