Jesus warns ONLY not to use miracles as "spectacles" to INDUCE BELIEF. Meaning, he does not want us MIS-USING miracle-working power to dazzle and delight onlookers just for the sake of achieving an EGO PURPOSE.
Ie this is a MIS-USE of the miracle, a misunderstanding of its purpose to heal and resurrect, and instead is being used to deceive and coerce and manipulate people.
Jesus only means, not to use miracles for shock and awe. They are supposed to be used to teach, to heal, and to prove that the ego's laws are not true.
And that DOES mean, that miracles CAN BE SPECTACULAR.
It simply means that you wouldn't "put on a spectacle" just for the sake of dwelling on how cool the miracle is. But this DOES NOT MEAN, "therefore.... don't do anything spectacular."
People have mangled and twisted this into a fear-driven lie that you should not perform any kind of spectacular miracle and that therefore miracles are not and should not be spectacular. That's NOT what Jesus has said or what He means.
And if he didn't want to do anything which people might be surprised by, he would never have been able to do ANY of the miracles that he did do.
Read what the Course actually says instead of believing people's second-hand mangled versions of it.