I diy my liquids and wondered that too. I could tell that newly mixed juices tasted different.
I started putting my freshly mixed stuff in a cup of hot water from the microwave. Repeat, to get the
juice fairly hot and thin, then shake it.
Now I can't tell a difference from freshly mixed stuff and stuff that's months old.
I've read somewhere that someone preferred the nicotine to be a slightly oxidized and a little brown. He said it gave it a better flavor. I guess it's preference. I like consistency.