Edit: In my case it's a bug with Cryogenic Tanks mod, followed the modder's instructions, deleted some plugins, and now contracts are working 100%.
Confirm, it's happening to me too. In my case, it's when I install Cryogenic Tanks & Kerbal Atomics. No need to have many parts unlocked in the tech tree, if I install these mods and start a new Career, accepted contracts will immediately dissappear when going into VAB, SPH, or launch pad.
I remove these two mods and contracts are fine again. I don't get disappeared contracts back, but new ones I accept no longer disappear.
