S9+ EE (UK), Android 9... I can't shoot videos in Pro mode either. This is a feature I miss greatly - I filmed a clip the other day at a press event, but I could only use Pro mode for photos. Videos I could only shoot in Auto mode - and every time I zoomed in/out, the exposure changed. So much for the AF/AE lock - which didn't work. So I uploaded the full clip to YouTube to show just how bad the video quality really is.
Appalling, really, that you can't shoot videos in Pro mode (a la Galaxy S7 & S8).