I think if a player quits and is replaced by a bot, they should not be
allowed to rejoin the game. I also think if a player is disconnected,
after a certain amount of time (maybe 20 minutes) they should not be
allowed to rejoin the game.
The reason why is exemplified perfectly in a game I played the other day: https://youtu.be/IOZOye3tpL8
There's a summary of the game in the video's description.
Players know that most other players will let a weak bot recover and get into positions they'd never let a human get into. They can take advantage of this by quitting or breaking their connection when they are weak so that the bot can take over. This is clearly cheating. If players weren't allowed to rejoin games they've quit, and/or weren't allowed to rejoin after 20 minutes of the bot playing for them, this type of cheating could be prevented.