Publicize: wpas_default_prefix doesn't apply for scheduled posts

> I wrote a tiny plugin for customizing my sharing message for jetpack publicize:
>
> ``` php
> add_filter( 'wpas_default_prefix', 'add_default_publicize_hashtag_prefix', 10, 4 );
>
> function add_default_publicize_hashtag_prefix() {
> $default_tags = 'New on #SHORTSCORE: ';
> return $default_tags;
> }
> ```
>
> It works in the backend:
>
> http://s23.postimg.org/vgaxvt42j/Bildschirmfoto_2015_03_10_um_09_39_33.png
>
> But it does not work for scheduled posts. Facebook and Twitter do not show the prefix.

Reported here:
https://wordpress.org/support/topic/wpas_default_prefix-doesnt-work-for-scheduled-custom-post-types-with-publicize?replies=1&view=all

Start by tracing the wpas_default_prefix filter through the Publicize code paths for immediate and scheduled posts, using the reported support case as context. Reproduce the difference between backend publishing and scheduled publishing, then verify that the custom prefix appears in the Facebook and Twitter messages for scheduled posts.

Written by the indexing model from the issue text.