Low solubility of propanal compared to propanol, propanone, propanoic acid and methyl ethanoate
They're all on the verge of miscibility, methyl ethanoate (methyl acetate) is in fact not miscible according to wikipedia (soluble to 25%).

The acid is the most polar of all the C3 compounds you name. Butyric acid (C4) is also miscible with water, but valeric acid (c5) already not.

n-butanol is already rather badly soluble (8g in 100ml) in water. Same with butanone, etc.

Compared to the ketone, the aldehyde has a longer unpolar part, that is overall less influenced by the polar carbonyl bond. Both are clearly, i think, the least polar of the group, and so the aldehyde just drops beneath the line.

No mystery here. ;-)