I've had this happen to me constantly! For me, it seems to happen after completing 1 or 2 missions/contracts. I typically play at x1 play speed unless I'm waiting for drops to grow. So when I go to accept new contracts this is very annoying.
I also have it happen many times where I accept a contract (leasing vehicles or not) and the contract disappears. I check the field for the work that was supposed to be done, but nothing happened to the field - no in-game work was done. Very frustrating!
I've also seen it where i start a contract, save the game, come back to the game later, and the contract is failed.