When using flatpickr with alt input (for pretty formatting but submitting a conventional date, e.g for Rails), the form-control looks like a disabled input. This also fixes the appearance of validation states once a form has been validated.

Flatpickr styles for Bootstrap4 - validation & standard appearance

.form-control.flatpickr-input{
  &:disabled, &[readonly]{ background-color: #fff; }
}

// Be explicit on aria-invalid instead of pseudoclass :invalid / :valid
// Will allow error state to show on inputs like select2 and flatpickr
.form-control[aria-invalid='true']{
  .was-validated & + .flatpickr-input,
  .was-validated &{
    border-color: $form-feedback-invalid-color;

    &:focus { box-shadow: 0 0 0 .2rem rgba($form-feedback-invalid-color,.25); }

    ~ .#{'invalid'}-feedback,
    ~ .#{'invalid'}-tooltip { display: block; }
  }
}

.was-validated .form-group:not(.has-error) .form-control + .flatpickr-input { 
  border-color: $form-feedback-valid-color;

  &:focus { box-shadow: 0 0 0 .2rem rgba($form-feedback-valid-color,.25); }

  ~ .#{'invalid'}-feedback,
  ~ .#{'invalid'}-tooltip { display: block; }
}