Same here. They work for about 30 seconds to a minute (maybe) when starting a video, and then stop. Chrome 73.0.3683.86 on Win 10 (also 1809).
Edit: It seems to be that what triggers this for me is having a video in either full-screen or "fill browser" mode, and clicking on the playing video with the mouse (thus pausing it). After that, no keyboard controls will work until the page is reloaded.
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