Custom URL that should be used to redirect after publishing a Landing Page in Page Builder is ignored. This behavior should be aligned with the one introduced in AdminUi:
https://github.com/ezsystems/ezplatform-admin-ui/blob/1.5/src/lib/RepositoryForms/Form/Processor/Content/UrlRedirectProcessor.php#L64
to make allow redirecting if redirectUrlAfterPublish is specified, ref: https://github.com/ezsystems/repository-forms/blob/4c001b5847c93d61184dfadfce3998cd593520b0/lib/Form/Type/Content/BaseContentType.php#L44

Instead of customizing the default form in Page Builder the easiest way to test this behavior is changing value attribute to some external URL of ezrepoforms_content_edit_redirectUrlAfterPublish input in the edit view of Page Builder, as follows:

<input type="hidden" id="ezrepoforms_content_edit_redirectUrlAfterPublish" name="ezrepoforms_content_edit[redirectUrlAfterPublish]" value="http://your_url.com">