The split tool is a bit weird when editing lanes. If more than one stacked event is selected all stacked (even unselected) events are split when clicking one of the selected events with the split tool.
This also applies to tracks within folder groups (Even with group editing off). If two events (or more) on two different tracks are selected, all events on those tracks are split when splitting one of the selected events.
Selecting one event is a bit like selecting all events if you split one of the selected events. But, if you split one of the unselected events it splits just that event (and corresponding events on other tracks if you are group editing. A correct behavior.).
This is really unintuitive and feels like a bug. Moving and sizing events works fine, it’s just the split tool that is affected.