Is this some kind of joke? Why is it done this way?
i think because if the bot gets damaged and you replace it by joining, then you have about the same level of corruption and health that the bot previously had. which honestly is kinda fair and is intended.
otherwise if people or new players would constantly join with maximum hp, it would make scenarios kind of easy.
It is unfair to those who join the game in the heat of battle with low health and under the effects of corruption. They did nothing wrong 
AFAICT, it’s much weirder than that. I regularly have damaged bots get replaced with players that have full health. My current suspicion is it’s related to replacing a human player who has quit.
As to why? Who knows if it’s even intended.
im pretty sure that they talked about it in a devblog, cause they wanted to stop people that were rejoining back and forth abusing of the rejoin (even more cause with the mod) to get saved by themselves from nets and ledge and hounds grabs, which i think it still happens
yea maybe, but i reckon bots counting too but i dont know how fully works this
i think if you a new player joining and all bots are like hogtied and there is still a human, then i think maybe you spawn with full hp but not too sure.