I had this problem too and contacted Humbrol. Their response was:
Every batch is tested for sheen level so know that the product is within spec before filling the substrate. We can only reiterate that it is important that the product is thoroughly mixed before use
The satin and matt will contain silica which will tend to settle down as the white settlement shown. It is important for the product to be shaken well before use.
Regarding the use of the clear, I would suggest the following
HUMBROL CLEAR SHOULD NOT BE APPLIED BELOW 15 DEGREES CENTIGRADE OR IN A DAMP ATMOSPHERE. THE PRODUCT SHOULD ONLY BE USED ABOVE THIS TEMPERATURE IN A DRY ATMOSPHERE. IF APPPLIED IN A DAMP ATMOSPHERE AND BELOW 15 CENTIGRADE THEN THE FILM WILL DRY WITH A MILKY APPEARANCE.
We have contact the supplier and we are with the help of a toxicologist looking to improve the formula and stay within the regulations .