@andyrpsmith I see it in VP22 a lot, esp if I've cut something & haven't zoom out to refresh the timeline, it's like the thumbnails don't update whilst it's playing or if they're not seen on screen..
The short answer is that the preview icons are not generated because they're trying to keep the playback speed up. Once you stop the playback, the icons will be generated as that processing time is no longer needed for playback. This also was changed in previous versions - it didn't start in 23.
Many thanks jetdv, in the link to the other post Vegas staff explain the issue and that in the future they will look at the balance between timeline refresh and preview performance. I do not see this in V22 on my PC at least.