This may be related to Composer on mobile is fullscreen even when keyboard is closed.
When viewing the composer preview, you can see the page behind it, as it doesn’t go all the way down (if the composer is intended to be fullscreen):
Scrolling down more (larger gap):
You can actually click on the topic through the gap:
Consequently, you can see the topic behind it:
To scroll, I’m scrolling on the big blank space below the Create Topic and pencil icon buttons. That causes the page behind to scroll as well, although the preview doesn’t. However, you can scroll the preview if you scroll on the actual preview area (i.e. above the said buttons).