In an attempt to get the Galaxy S9 and Galaxy Note 9 Android Pie update out there as quickly as possible, Samsung may have forgotten to include the option to record videos in Pro mode. On Android Pie, video recording gets a dedicated mode in the camera app. It's a change many haven't taken kindly to, and it seems Samsung didn't take into account that this would require an important change in Pro mode as well.
Before Pie, video recording had a dedicated button next to the regular camera shutter button, so one could simply switch to Pro mode and hit that button to start recording videos with manual camera settings. But Pie has a separate Video mode, so switching to Pro mode only allows you to take photos. And, funnily enough, if you go into the camera settings and try to modify what camera modes you see in the camera interface, the description for Pro mode says that it lets you “take control of your videos.”
It's clearly a major oversight on Samsung's part, and the company seems to have rushed to make the camera app more iPhone-like without thinking things through. And it also raises questions about how final these official updates really are. While the Galaxy Note 9 has received the final update in only one country, the Galaxy S9 and S9+ have been updated in quite a few markets. The problem exists even on the newer Pie build a couple of countries have received.
Whether the worldwide rollout will fix this remains to be seen, but as it stands, you might want to hold off from updating to Pie if you like taking videos with custom camera settings.