This was similar to my own experience described earlier, however I should add that when I placed the bottle in the freezer the liquid froze, something vodka never does.

I was only able to reverse it by mingling the filtered vodka with regular vodka and even then I had to try twice, ie. add enough of the non-filtered stuff (maybe 1/3rd).

What I am wondering is whether the filtering may trap some ethanol molecules and could the smoothness possibly derive, at least in part, from a greater water content?