I can only add to JimC's speculation. It does appear to be under the surface and all I can think of is possible contamination like skin oil or a solvent of some kind. BigDaddy makes a good observation by comparing it to blushing as well.
I have not encountered this problem with polyurethane either and don't have any solid diagnoses or fix.

However I will raise a point about trying to achieve the satin finish as you mentioned.
I don't want to sound negative or tell you you shouldn't do it, but turning a gloss finish into a satin finish is never going to look like a satin finish. It will always look like a glossy finish that was de-glossed.

This is because a genuine satin clear coat has a small proportion of solids mixed into it that actually diffuse the light and give it that non-shiny appearance. Very much the same way that a satin finish can be polished to a gloss, but it will lack the depth a real gloss finish material (poly, nitro etc) has. It's merely shiny "on top".

As for feel and touch (for instance on a neck) yes, de-glossing will feel similar to satin, but again side-by-side comparison there will be a visual difference.