Like the first game, Total War: Warhammer 2 will be receiving balance and bug-fixing patches throughout its lifespan. The first of these now looks like it will be concurrent with the upcoming release of the Mortal Empires campaign.
That’s the indication given on a thread from the Creative Assembly forums about the persistent bug that stops units fighting in the middle of battle. ‘CA_Matthew’ confirms that this Total War: Warhammer 2 bug is now fixed, but also states there won’t be a hotfix ahead of the release of Mortal Empires. Doing so, he says, would affect the launch date. He says “we will be deploying it for patch 1”.
So, from that we can infer that Patch 1 for Total War: Warhammer 2 will not come before Mortal Empires. It doesn’t absolutely confirm the first patch will release alongside it either, but that seems fairly likely from the information provided.
There is no official date for Mortal Empires yet, though 26 October has been confirmed as “one of the dates” Creative Assembly are considering. That’s probably a case of whether it’s ready to go at that point, or needs more time.
For the sake of comparison, the first Total War: Warhammer was released on 24 May and received ‘Patch 1’ on 29 June. Seems like the initial patching schedule will be roughly the same for the sequel.