Exploit?

It is, as you say, a bug with the game over which the affected player has no control, so no, you're not going to get in trouble for it. Although to be fair to the other player, it can look like a hack to someone who has never seen it before.

To begin with, reporting someone in the endgame chat is very unlikely to get someone banned for something like hacking. The team usually requires video evidence in order to enact any kind of punishment. The devs are also very careful with their bans, and they won't ban someone if there is any doubt at all that they might be innocent. Not to mention they are almost certainly aware of this bug. So even if he did have video footage and sent it to them to report you, it still wouldn't do anything because the Support team would instantly recognise it for what it is.

So don't worry - you didn't do anything wrong, and nothing bad is going to happen to you or your account 😊

Also, if he's harrassing you in other games, depending on how he goes about it that is actually against the rules, so you could report him for that if you chose.