Please, try this:

within an article, open Conditional Content

Go to the Content tab
Go to the Visitors → Devices rule
Select Mobile

Go to the Content tab
Type OnlyForMobile

Go to the Alternative Content tab
Type ForWhateverElse

Click Insert

In the article, only {show devices="mobile"}{/show} has been inserted.
No content at all. No {show-else} condition.

If I insert condition and content by hand, everything works perfectly. But it sounds strange to me. Am I doing something wrong?

J ‎5.0.1, PHP 8.2.9, Ubuntu 23.10, Firefox 120.0 (64 bit)

Fixed.
Thank you so much!