Change the function in the datepicker component to this.

[KarelJanVanHaute]

private async initDatePickers(pickers) {
    const flatpickr = await import('flatpickr');
    switch (lang) {
      case 'nl':
        flatpickr.default.localize(Dutch);
        break;
      case 'fr':
        flatpickr.default.localize(French);
        break;
    }
    Array.from(pickers).forEach((picker: HTMLElement) => {
      const enableAttr = picker.getAttribute('data-enable');
      const enable = enableAttr ? JSON.parse(enableAttr) : undefined;

      flatpickr.default(picker, {
        ...(enable ? { enable } : {}),
        onChange: function (selectedDates, dateStr, instance) {
          instance.input.dispatchEvent(new Event('check-validation'));
          if (instance.altInput) {
            instance.altInput.dispatchEvent(new Event('check-validation'));
          }
        },
      });
    });
  }