Placing a Factorissimo building in the BP Sandbox and then entering, won't allow you to exit.

Entry is by moving to the "door" and triggering the entry. Exit should be the same functionality, but it doesn't allow you to actually exit. The player is stuck inside the Factorissimo surface.
Also unable to select the "Toggle Sandbox" button to "force" eject the player.

For entering and exiting, both are up to Factorissimo 3. The last time I checked, it still worked, but it was clunky. You might have to interact with some things near the door. In any case, bring it up to that author, not this mod.

For being prevented from exiting the Sandbox as you normally would, that is simply how these mods must work together. You aren't in the Sandbox, you're in a Factory.

Shift-click on the substation to enter/exit the factory remotely.