I have noticed the same thing re saz 18. After a few minutes in the glass it does seem to gain a little bit of weight and nose back but not as much as when it was first opened. I also find that the Glencairn really helps the saz 18 where you might lose some stuff if consumed in a standard tumbler.
Having the VWFRR open right next to it isnt helping its case either. I opened a fresh bottle from the 2011 release last week and now its almost half gone!