For anyone else running into this, you can workaround it by:
- open the file C:\Users<your pc username>\Games\Age of Mythology Retold<your steam id>\users\UserProfile.xml
- Find the line:
<lastselectedciv>1</lastselectedciv>
- If it’s 1-3, change it to 4
- If it’s 4 or higher, change it to 1
This lets you start the game with a different menu scene, which somehow fixes the bug.