fields added in hook on buildFormContent are empty

[valan]

This issue appeared after upgrade from 2.7.2 to 2.8.35

There are two hooks:

- One triggers on ProcessPageEdit::buildFormContent where I add custom fields, e.g. like $form->insertBefore($virtualfield, $realfield);.

- Another triggers on saveReady where I get and process values entered in these 'virtual' fields (e.g. $page->virtualfield).

Before upgrade it worked. After upgrade, my custom 'virtual' fields are always empty in saveReady hook.

Please advice what could be a reason and how to fix it.