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…
It was a problem on my device (Galaxy S21) a year ago, but an Android update ended up fixing it for me later on. It seems to vary based on which version of Android the app is running on.