Under the Model tab in Roblox Studio, there is an option to toggle automatic surface joint creation.
When this toggle is disabled, joints are not created when using the Workspace’s JoinToOutsiders function during run-time.
I observed this behavior in my building game. My constructions would break apart upon loading without the option checked.
Thanks for the report. Looks like this happens with MakeJoints too. It also looks like this was still an issue before the most recent surface join changes, but I should have caught it when making the change. Unfortunately we won’t be able to have a fix out until January.
Note that this will only happen in studio during simulation and if you start simulation with it disabled. If you start with it enabled, then turn it off while running, JoinToOutsiders still works. There also shouldn’t be any issues in a live published game.