I think this happens when:
1) You have an event with a 2nd Keyframe (all events have at least one keyframe on the first frame)
2) You split that event and delete the beginning.
I think if you drag either end to shorten an event, the keyframes adjust and cause no problem. But if you chop the beginning off .. problems can occur with keyframes.
This is especially true for titles. Shorten a title by cutting off the other end, not the beginning, or drag either end to shorten the event.
Am I making sense? Sort of hard to explain. But, in your case, hopefully “lesson learned” ..