TYPO3 Core

This behavior is correct. requestUpdate must reload page to reveal new fields when something is changed. Same behavior for "type" fields. Otherwise no fields will be shown if any required field is missing.

I know that requestUpdate saves the record. But this leads to uncomplete / unusable records when required fields are empty.

So why must the record be saved? It could be possible to reload the form with the new fileds without saving the record to DB.

Behavior is the same as for "type" fields. When field is updated, displayCond may trigger other fields to be shown. For that we need requestUpdate on the field.

Changing "type" field (for example, page type from Standard to External) also saves the form without required fields. But form will still show that fields must be filled in.

Yes. But problem is the following:

After a updateRequest the editor might stop filling the form and close it or click any other view. Remaining is the uncomplete record.

So my question is only: why is the record saved? The filled fields should be in the POST-vars so they could be refilled again without saving the record.