The Sequencer timeline can't be scrubbed with the arrow keys because the Sequencer Window loses focus after an arrow key is pressed. This is a regression from 4.21.2 (CL-4753647) where repeated arrow key presses will scrub the timeline continually.
This was reported and tested in 4.22 Preview 7 (CL-5531554). This was reproduced in Main 4.23 (CL-5552558)
Results: Nothing happens
Expected: The timeline moves one frame to the left or right again
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-72001 in the post.