If you use "split equally" remove the checkbox-feature.
I came across multiple people who put expenses as split equally but unchecked themselves from the expense. It's a very sneaky thing to do and you have to manually check the expense. So if you add people to an expense and you choose to split equally the ability to remove people should be more clear, like putting a warning on the activity like "split equally among unequal people" or something.
Sorry for the trouble! I can see how this would be easy to miss if used maliciously, though it is also an intentional feature. (In some groups, it’s quite common for people to add bills that don’t involve themselves.)