I own a Nakayama Maruichi hone. I'm not quite familiar with the terminology, but I believe it's not the same as an Asagi.
Anyway, with every single stone I raised a slurry on so far (that includes over a dozen of coticules, several Belgian Blue Whetstones, but also the Naniwa Chosera 10K, and my Nakayama) I have noticed, too at least some extent, the "slurry-dulling" effect.
The approach Bruno mentionned, gadually washing away the slurry as the honing progresses, is a great way to work around is.
I can't speak for some other hones, but still, I'm starting to think the effect is inherent to honing on slurry. Of course, the severeness of the effect depending on the nature of the hone.
Just a hunch.
Bart.