I have had a similar problem in the past and I seam seal a bunch of tents on an ongoing basis. I had a partially used tube of GE Silicone II and mixed it with Klean Strip Odorless Mineral Spirits and tried it out on some spare pieces of sil-nylon as well as some caulk right out of the tube. The mixture never seamed to dry and had an oily sheen on the sil. The raw caulk never cured normally, was still wet after a few days and when it did dry, it did not adhere to the sil very well. I went out and got a new tube of Silicone II and used the same bottle of Klean Strip and did the same tests with much better results. The mix dried overnight and the bead of caulk cured and dried normally. This obviously points to the Silicone II as being the culprit but not sure if it had just been sitting around too long, was contaminated in some way, or what the problem was with it. I normally use a closer to 1:1 mix because I can control it better, but I don't think this affected the results.
I hate to disagree with everyone above (OK, maybe I don't) but I think it was the caulk. Try a similar test to prove it.
Now, what to do with the Duo-Mid? Not sure of the best solution here. If you are happy with the results of toweling off, go for it. Try some tests on a spare piece of sil and see if recoating it with a "good" mix will cover over the problem (before or after toweling off the residue).
I hope this post helps as I feel indebted to you for many of your insightful and information rich posts in the past.