In the event anyone else runs into my issue, with the help of support I figured out what happened.
I played through the whole game without using cloud saves and never signed into the Xbox app it wanted me to use. So all save games were saved in "C:\Users\PROFILE_NAME\Saved Games\The Outer Worlds Windows 10\__LOCAL__" . After installing the expansion and logging in to the game it bugged me about the Xbox app and cloud saves again so I logged into that. At this point I believe the game changed where it looks for saves to "C:\Users\USER_PROFILE\Saved Games\The Outer Worlds Windows 10\MICROSOFT_ACCOUNT_ID". This location was new and empty so the game saw no save files. Simply copying my saves from _LOCAL_ to the microsoft account folder allowed the game to see them all. And bam, problem solved!