A phenomenon in IOS can be the lack of box-shadow shadow. There is no further indication of the CSS border-radius. If this is set to 1px also the box-shadow appear immediately and correctly. The radius must be at least 1, for example,
box-shadow: -20px 15px 15px 1px #A0A0A0;
border-radius: 1px;
Otherwise, the definition will be filtered out as defective and not taken into account.
To have to find this simple explanation, a little time has passed, maybe it will help someone else on.
Click here for the definition of box-shadow of w3.org