If you start a new game do you still have this problem? You would need two controllers to start a game and join in split screen to make two characters right away (then after getting in-game disconnect the second controller), or create one character and skip through the tutorial as much as possible to get to Fort Joy to recruit one or more companions. While a new game on Story difficulty would be the easiest to test getting to Fort Joy, if auto saves are enabled, or quick saves used, they could overwrite saves from your current playthrough. If you don't already have an honour mode difficulty save, starting one would avoid that problem, since it uses a separate, single save slot.
If you start the game from a different PS4 account, if available, and start a new game, do you continue to have this problem, when you can test with multiple characters? A difference account would give a separate drive space for saves.
Please email supportdos2@larian.com with your latest save (or a Dropbox or Google drive link, etc), and if available the most recent save where splitting and joining characters works properly and the oldest where it does not. The saves would need to be transferred to a USB flash drive first.
In the Saved Data in System Storage section of the Settings menu, click Copy to USB Storage Device, select the relevant save(s) and Copy. From there you can zip the files, then copy to email or upload somewhere and provide a link.