I have had the same problem several times and saw on other threads here that it is in fact a problem with default persona loading, and still not fixed as of today. You can solve the problem by selecting new game in the main menu and setting all the same opponent avatars to the same slots/color as they were in your current game that has the wrong personas. Then you dont need to start the new game, just set it up then exit to main menu and load your current game. Note that it probably only works with the base avatars, as all the others personas (twitch dorps, collection on G2G) avatars might still shuffle because they seem to have a relative position in the list contrary to the base ones that are absolute. And I am still not sure if continuing the game and resaving it with the wrong avatars can be resolved the same way after.