As far as I’m aware, there are two situations where you might be able to hear content from the main timeline while the cursor is in a transition timeline.
The most likely is that one or more of the sound modules overlapped by the transition marker is of a type that does not cut off when the cursor leaves it. Multi sound modules not set to loop and event reference sound modules are the most likely candidates, but almost any module with an AHDSR modulator will also continue for a while after being left by the cursor. In these cases, you need merely alter the properties of the sound modules in such a way that they cut off once left by the cursor. (We’re planing to expose cutoff behaviour as a separate property in a future release, but for now it continues to be based on a sound module’s other properties.)
The other possibility is that your transition timeline features a “source region” in which it plays content from the main timeline. If you open the transition timeline and can see content from the main timeline inside it, this is why. (This is particularly common in projects imported from certain older versions of Studio that did not support customised source regions.) If this is the cause of the behaviour you’ve observed, all you have to do is click and drag the right edge of the transition region to the left until it disappears. Note that you will need to do this for every track of the afflicted event.