A requirement is that the user cannot select a value different from those in the list, that is, when the user, for some reason, writes something in the multiselect, an error message is thrown, or else, that it does not allow the text input.

While disabling the "Allow free-form input" option allows input to the multiselect not to be counted as an item, it still allows free typing within the multiselect.

With this option selected, the user input inside the multiselect is not considered a list item until the user clicks on it in the dropdown list, meanwhile, the user input is not an item, so using an activity that iterates through the list is ruled out, since what the user writes isn't added to the list until they click.

So blocking the write input to prevent the user from typing something into the multiselect seems like the only viable thing to do, though I haven't found a way to do it.