Beer has off flavor, sample taken while bottling tasted good

I brewed an IPA about 3 weeks ago. Everything went fine during fermentation and I've dropped a couple of hop pellets 4 days before bottling into the fermenter as I planned to do.

When bottling everything went fine and I took a sample to measure the FG (with priming sugar, forgot to take it before).

I've put the sample into the fridge (after measuring the FG) and drank it after 3 days, it tasted good! I decided to put a couple of bottles into the fridge, and when taking them out of the closet they were in I saw the following "dust" swimming inside the beer (picture at the end of post).

When tasting the beer it lost its juicy taste that the sample had and it looks a bit darker. From my experience I would say it has this "old" IPA taste and sugary/caramel smell.

I've put the beer into a closet for 4 5 days once bottled, temperature was maybe a bit high (probably reached 80°F at some point).

My question is: Why did the sample tasted great and the actual beer not as good? Was the room temperature too high? Did I leave too much air in the bottles? Does it have anything to do with this "dust" in suspension?

Hope some of you can help me understand what I did wrong. My other all grain beers went fine but I didn't do any dry hop.