Brian Gaff wrote:

surely it should not go solid as the pressure is pushing out not
sucking air in,

In the past, I've thought storing a part-used can inverted might be
better, as the gas couldn't leak out without it being noticeable, but
apparently storing upright is the correct way.

unless the cleaning of the nozzle made it leak the
propellant out and the air then got in.

some foam (probably dissolved by acetone to make a thin "glue" before
the acetone evaporated had created a seal around the nozzle on the can,
I had to press it firmly to break that.

Sounds like that could be dangerous as it expands and might then presumably
burst out of the can.

The can isn't under any extreme of pressure, it has some give when
gripping it firmly, the contents aren't fully solid, but it seems
there's a "plug" of cured foam above the liquid and whatever gas remains.