If it is a surplus bivvi, expect anything.....
Or even a preowned item.
But it is my experience goretex doesn't always work, but I understand even gore tex has to be proofed now and again to bead the water on the surface so it water can run off and allow the perspiration from inside to pass through. Nikwax contains silicone which is a hydrophobic chemical and it allows water to bead and run instead of splotch and spread into the fabric up to the gore tex membrane there inhibiting perspiration transmission. Also gore tex does not work in certain conditions and I have found where the ambient temperature outside is high, perhaps higher or equal to that of the inside gore tex action can stop or even reverse.
But as to what can be done for now, well if you are lying in the damp patch, put something impervious over it like a karrimat if you have one with you to insulate you from the gore tex and the the damp and sort it out later when you are at home.
But yes a detergent residue on the fabric will inhibit the action, that is why nikwax offers the tech wash for such fabrics- as detergent breaks the surface tension of water and any residue will continue to do so, as detergent residue often remains even on well rinsed fabric, ad the rule is, if you can smell detergent when the fabric is dry, the detergent is still present.