Is it considered a sin or an act of disbelief to place the Mus-haf on a dry bedsheet that has an unidentifiable trace of madhy on it, and is that forbidden?

It is good to alert you that a defiled mattress is not purified except by washing it completely. If you were certain that the mattress would fall on the Mushaf (copy of the Qur'an) and you did not lift it, then you have committed a sin, and you must seek forgiveness and repent. The pleasure of Allah must be given precedence over the pleasure of creation. It is not permissible to place the Mushaf on a defiled mattress, out of reverence for the Book of Allah. Placing the Mushaf on a mattress is not apostasy, unless it is done with the intention of belittling or disdaining it.