Sorry for adding a "me too" but I just wanted to share my experience. I have an Apple Watch Ultra 3, watchOS 26.0.2 and AirPods Pro 2s. Cellular is activated on my watch and I do not workout with my phone while running. I have 1 mile splits activated and have for years so I know what the right settings are and I've changed nothing. Workout buddy is disabled because last I knew, at least with my original Apple Watch Ultra, it could not function without the phone.
The voice notifications for splits are absolutely sporadic at best. Seems like most frequently, there is no audio notification at all - my watch will pop up a notification for each split but no audio cue. When it does play, it's a toss up as to whether it stops my music to do the notification or just lowers the volume where as it used to always just lower the music volume.
Extremely frustrating to have such inconsistent behavior as it's no longer a device that can be counted on if you are really counting on that split information for your training - luckily I am not that serious about it but I can understand how someone would be.