It looks like someone already discovered what the cause of this was on github. There's a pull request that fixes the problem, but it has yet to be approved:
Pull requests · le-doux/bitsy · GitHub
Looks like when 8.6 was released the devs made a typo that caused the x coordinate to be used for both the x and y coordinate of the exit
You can fix this in the browser yourself by opening the dev tools and evaluating the below code on the console (basically patches the running code with the fix).
function createExitData(x, y, destRoom, destX, destY, transition, dlg) { return { x: x, y: y, dest: { room: destRoom, x: destX, y: destY }, transition_effect: transition, dlg: dlg, }; }