On my pixel whenever I use the pull to right gesture to go back on a post the background will be black. However, if I use the back button or system back gesture the background shows the previous screen.
On my tab both gestures show the previous screen, so it’s not a general thing I guess…
Was reported months ago but LJ is MIA.
He’ll be back eventually and do a big update. It’s not ideal, but this is how he always operates.