According the documentation that's the desired behaviour:

https://www.vmix.com/help24/ReplayEvents.html

In my opinion, this behavior is not ideal. If you want to play multiple events and only one event with a different speed you have always to modify two events. The one with the different speed and the following one to set the speed back. And you can't see at event level, which speed it uses. You always have to check the above events if the speed is changed somewhere. I also see some problems if someone starts reordering the events. Then the speeds get completely mixed up.

It would be better if the event speed would only apply to the respective event. Events with "---" should always play with the overall speed selected.

Best, Patrick