I'm not sure exactly what you mean that it "gets brittle when it's already dry". Are you talking about fondant figures or cut outs?
I really don't know. Can you post a photo so I can see what your talking about? I have flowers that I made from fondant months ago so I'm trying to figure out why you're having this problem.
If you add tylose to fondant, it really just makes the fondant dry faster and able to be rolled thinner.
I don't see it as adding real strength to the dried product.
If your fondant piece was too brittle, it may have been rolled too thin for it's intended purpose or the pieces may not have been rolled out without lines/cracks in it, leading to breakage at those spots when it dried.
HTH
Rae
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