Starting COOP with bot in Rocket League

This answer is for PS4, but I expect it works similar on PC.

While in the main menu, there is an indicator in the lower left corner telling you which local players have connected already. It also tells you "Press options to join". After every local player has joined this way (pressing the options/start button), you can simply start an exhibition, seasons or online match (except ranked) and your couch buddies will join automatically.

Tip: You can also disable bots by selecting "No bots" as difficulty in the match settings.

There is no reason for bots to actually do that since they're specifically designed to play along with you. So a bot sitting in the goal box and not going after the nearby ball is definitely a hangup in the bot's path logic or something related to that.

It's very likely that Psyonix's AI update set some things back. This happens to pretty much everyone designing AI because there are so many factors that go into intelligent play.