So reloading a Fort Joy or Lady Vengeance save gets everything working again?
Try lowering the graphics settings and resolution, and switching to Windowed or Fake Fullscreen display mode, to see if that will make a difference.
Try exiting out of the Steam client, and starting the game directly from the '..\SteamApps\common\Divinity Original Sin 2\DefEd\bin\EoCApp.exe' program file, by right clicking and running as administrator (or '..\Divinity Original Sin 2\Classic\EoCApp.exe' for the original release).
Try verifying local files: in the Steam library, right click on the game and select Properties, switch to the Local Files tab and then click on the 'Verify Integrity of Game Cache...' button.
With the GOG version, in the (optional) Galaxy client, select Divinity: Original Sin 2 in the left column, then click on the More button, and in the Manage menu select 'Verify / Repair'.
If you load your last save on the Lady Vengeance and go through the act transition again, does that fix the problem?
If that doesn't help, please email supportdos2@larian.com with your latest save (or a Dropbox or Google drive link, etc), as well as your most recent save before leaving for Reaper's Coast. If you are using any mods, also include a list of those enabled and/or the modsettings.lsx from your profile folder.
Each save is a folder in the '..\Documents\Larian Studios\Divinity Original Sin 2 Definitive Edition\PlayerProfiles\<ProfileName>\Savegames\Story' folder.
In Windows Explorer you can zip a file or folder by right clicking on it/them and selecting 'Send To | Compressed (zipped) Folder' (and similarly in OSX/Finder).
This is not a known issue, that I am aware of.
