Date field value is always empty

The issue is following:

I have two fields - Date field and Select field.
The Select field has 3 options:
1. A
2. B
3. C

When B is selected, the Date field should become required (and it works without problem). The problem is that even if I fill date in it, it still sets error that the field is required (treats it empty always).

Found where the problem is in conditional_fields.api.inc

Actually, $dependent = $form['#conditional_fields'][reset($element['#array_parents'])]; returns:

$dependent = {array} [2]
 field_parents = {array} [4]
  0 = "field_deadline"
  1 = 0
  2 = "value"
  3 = "date"

And the $form_state->getValues()['field_deadline'] returns:

field_deadline = {array} [1]
 0 = {array} [1]
  value = {Drupal\Core\Datetime\DrupalDateTime} [12]
   dateParts = {array} [6]
   formatTranslationCache = null
   inputTimeRaw = ""
   inputTimeAdjusted = ""
   inputTimeZoneRaw = ""
   inputTimeZoneAdjusted = ""
   inputFormatRaw = ""
   inputFormatAdjusted = ""
   langcode = "en"
   errors = {array} [0]
   dateTimeObject = {DateTime} [3]
   stringTranslation = null

So, the line 312:

$input_state = NestedArray::getValue($form_state->getValues(), $dependent['field_parents'], $key_exists);

is always NULL, since the keys are not the same.