I've never had Auchentoshan so can't comment on it myself, but are you familiar with the concept of the neck pour?
What is a whisky neck pour | Whiskipedia
Some say it's a myth and can be explained by other factors, but I'm fairly sure that first pour does taste a little different to me. Did you let it sit in the glass and rest for a bit before trying it?
Huh, interesting!
I always let it sit in the glass for a good 10 minutes anyway (purely because that's what my dad always insisted om doing).
To be quite honest it could just be that my palette isn't refined enough or that it just changes day to day, but I've found very few whiskeys stay consistently 'on point' for me glass to glass. I'm drinking that same Auchentoshan right now and finding it reasonably enjoyable, much more so then I did even just a few days ago. The only bottle I've finished so far and thought that every glass was at the very least consistently good was the Classic Bruichladdich.