Just want to chime in here that this was happening to me on version 5.5 when I was piloting my Sequencer camera using a second viewport (Window → Viewports → Viewport 2) set to ‘Cinematic Viewport’ (Perspective → Cinematic Viewport). Changing it back to ‘Default Viewport’ seems to have fixed the issue, but it may just be a matter of time. Thanks to @BOB.cat for mentioning multiple viewports.
As a side note, every time a save happens all of my sequencer actors in the viewport move positions, resize, or completely disappear. Moving to the next frame in the timeline usually helps, but sometimes I have to switch the second viewport from Default to Cinematic and then back again to get the media plate to behave. That is only worth noting here because switching to Cinematic view is (seemingly) playing with fire in the second viewport.