I highly doubt a fresh mix could burn off your taste buds, and would not worry about using a fresh mix-although even me liking fresh juice I do give it a very good and thorough mix before using.
I have had vapers tongue in the past, but only when I vaped heavily and some menthol sorted it out (Which was also fresh).
It is very strange though your juice tastes good immediately after mixing, but within a day it does not taste so good. That is a very short amount of time for anything to happen, so I really am stumped what is going on there to be honest.
I am glad as
@Huckleberried said though you are getting it more narrowed down, and I hope soon you have it figured out what is causing the problem.
You mention you get on fine with premium juices- you are really making your own premium juice, so it is very strange what is going on. They do have sweeteners and other things added, but you can add these yourself, I like capella supersweet, really does make some flavours better.
I notice your coil has had quite a bit of use though, do you have a new coil to try? This may well help, I find flavours to be much better when I rewick if I have used multiple flavours.
I hope you get the issue figured out soon and can enjoy your own DIY juice

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