Batches tasting different is not uncommon but for a batch to taste almost completely different is. Especially from a distillery like Highland Park. They normally will try and get every one of the main sellers like 12,15,18 to taste very similar, so their customers get something as close to what they are expecting as possible. I am really not sure what you can do, maybe email Highland park and let them know this batch is different from the the ones you have. Not sure what they will do but most distillery are very friendly and love to hear about things like this, so they can follow up if something is off.
I had a few bottles of Glenfiddich 15y taste slightly different batch from batch but nothing like Harsh or anything.Sorry to hear what your going though.
Edit-Just saw this review on Master Of Malt
Normally, I absolutely love this scotch, but the last time round, I ended up with a dud of a bottle. After surfing around online, I saw that others are complaining as well. My bottle was not a freak of nature or the victim of "cork rot." It was just a bad batch, so to speak. When the 18 year is good, it's very good, but when it's bad it's dismal. Beware of recent batches.