System info in which it was found -
MediaWiki version = 1.30.0
This same problem also exists on Wikipedia website too (read below for examples), so I should say the problem still exists in newer versions too (Wikipedia running on 1.32.0)
Info about phone screen -
Extension used = MobileFrontend
Theme = MinervaNeue
Please note that Wikipedia also uses these tools to create the mobile pages.
Important - Slideshow works fine in Vector theme in desktop screens.
Problem description -
When the following code (as suggested by MediaWiki documentation) is used to create slideshow it gives a slideshow in desktop pages using the above mentioned softwares. But it shows thumbnails in a column on phone screens using the above tools. The slideshow works when you select vector theme in phone screens too. So, the problem might be in the MinervaNeue theme or MobileFrontend extension.
Code (the usual wikitext used to create slideshow; nothing special)-
Pages involved -
Check the slideshow in the above page in both desktop and mobile screen and you will see the difference.