Use of a joining comma with a sentence with a single subject and two verbs

I’ve come across a few examples on this website, but I am confused as to why some of them are correct and others are not.

Apparently, the following is correct.

Agnes robbed the bank, and then she went out for a hamburger.

However, the following is not.

Ella wrote a statement for the media, and then screamed at her press agent for an hour.

While I do understand the justification for the latter being incorrect (It’s not actually two complete sentences as there is only a single subject), I don’t understand why this does not apply to the first sentence. Surely that only has a single subject as well?