Do you mean something like this:
Before a Event was triggered by scrolling.
After a Event was triggered by scrolling.
If this is the cause then make sure to make a Section like the exact big placements like what you have done with the Events that you have set them by scrolling into a specific spot. Like, make it "bigger". SMBX reconize only specific sections you made it that big like if you make it horizontaly but if a Event set on somewhere that isn't completly horizontaly like you can go up after a event was triggered and you go verticaly, it will break the background horribly at the spot you didn't scroll your level's boundary big enough where you set it on on event.