When editing a whole page (not just a single section) using the mobile wikitext editor, the collapsible sections should work in preview.

Once upon a time they didn't work at all. Recently, we accidentally made them work as a result of changes f083f3d7befb and da12ebf1d108, but the icons that indicate they can be toggled do not appear:

…compare to the icons in read mode:
(Note that the section edit buttons should not appear while previewing.)

To reproduce, visit e.g. https://en.m.wikipedia.org/wiki/The_Fighting_Temeraire#/editor/all (or any mobile page with #/editor/all at the end), and switch to wikitext mode – the feature is not normally accessible from the interface (see T203151), you can only access it like this or by switching from VE with changes.