I shot mine in a speed graphic outdoors and metered with a Seconic spot meter. The exposure times were in the range of 1/25th and 1/50th of a second. Could it be that we are seeing a speed / reciprocity failure problem. I'll warrant that you are using very very long exposures, a situation for which the paper is not designed, any more than it is for my rather short exposures.
IDK, but the speed I have been using is ISO 25 and my reference in the same camera is /was Polaroid at ISO 100 along with Portra VC at 100. The pictures included a MacBeth color checker as reference.
Oh, I also cut small sheets and placed them in an old empty Polaroid pack and exposed them in my RZ67 with the autoprism set at ISO 25. Same results. The one sitting next to me right now was exposed at f8 1/50". Scans of the negative and print are attached.