Irony is a phrase that when used its actual meaning is the opposite. I think its not irony that you are referring to.
The Jews were admonished for lack of faith earlier in the NT. By King David for example. Now through the extent of catholicism understood as universality, that any one can have access to God, not just the Jews, like Jesus Christ said “search and you shall find”, those admonishments for lack of faith apply not only to Jews but to Christians and pretty much all people, especially those who are somewhat under God’s wings, that is Christians.
Isn’t it odd then, or unfair, or unjust, that people who are baptized, receive communion and other sacraments from God one day they just leave Him, and join other beliefs or none, and teach people God does not exist even though they witnessed His presence in church but claim “they saw nothing”?
Plus Jesus knew exactly how things will end from His Father, in the Ghetsenami garden. It was no surprise to him at all. That is, for the case you presented.
For the general case, of Christians abandoning their faith for the next cool thing or just for nothing, yes it is sad and unfair. And the questions still stand. Isn’t it odd?