Question about latex paint which is solidifying in the can

I have five gallons of latex interior flat paint which I custom mixed from several partially full paint cans four months ago.
I went to use the paint last weekend and it had partially solidified in the bucket.
The bucket was well sealed. It wasn't the paint on the surface that was solid, it was a large core of paint with thinner liquid on the sides and top.
I did get the paint liquid enough to use. After painting, I transferred some into a 1 gallon container and into a cup sized plastic container with lid.
After a week of being sealed, the gallon and cup containers also appear to be solidifying from the middle out (or bottom up?).
I've mixed my own custom colors from leftover paint before and have never had this problem.
Anyone have thoughts on why the paint is solidifying so quickly?
Any way to retard or prevent it?
I want to be able to use the paint in a couple of years to top coat.
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It might be a chemical reaction of different brands, it is probably trash.
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