That is pretty strange considering the products you have used. Theoretically it should be fine. The hazing on the bottom edge and marks on the back actually look like reactions to something. Is it possible some other substance has found it's way onto the black before the clear? Alternatively the only other thing I can think of is it's been in contact with some kind of plastic that is outgassing softeners or something. Rustoleum seems to do a similar thing with certain foams.
If you stick your finger nail in an inconspicuous spot, does it leave a mark? If so, I would say that application may have been a bit thick and it's skinned over leaving the paint under uncured. Over time the solvents are making their way to the surface causing these issues.
EDIT: Do you ever get down Toowoomba way?
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