Using Chrome 66.0.3359.181, thumbnails might be split on two columns if they are inserted within a column.

Example: https://it.wikipedia.org/w/index.php?title=Parco_Natura_Viva&oldid=97414977#Parco_delle_estinzioni

The file is listed between Stegosaurus and Camptosaurus in the code:

*''[[Stegosaurus]]'' 
[[File:2 stegosauri.jpg|miniatura]]
*''[[Camptosaurus]]'' 

Using other Browsers (Microsoft Edge and Mozilla Firefox) the thumbnail isn't split.