Reproduced in:

• OSX 10.13.3 - Safari Version 11.0.3 (13604.5.6)
• OSX 10.13.3 - Chrome Version 64.0.3282.167

Steps

1. Edit a new page, I used https://www.mediawiki.org/wiki/User:JHernandez_(WMF)/ve
2. Type
   1. Hello
   2. <Enter>
   3. world
3. Go to the end of the first line (<C-p><C-e> for example)
4. Join the next line <C-d> (maybe Supr if your keyboard has it) and add a <space>
5. world comes up to the first line
   1. it has a smaller font size for some reason?
   2. Text is in the first line, says Hello world
6. Go to the end of the line (<C-e> for example)
7. Type <space>world again
8. Save

• Expected
  • The wiki page has the contents Hello world world
  • When editing the joined text has the same font size and behavior
• Actual
  • The wiki page has the contents:

Hello world world
world

More info

No runtime exceptions were seen in the console.

Published page

https://www.mediawiki.org/wiki/User:JHernandez_(WMF)/ve

In editing screenshot...

... of the weird smaller font size word when joining the line

Other failures

This happened to me while editing a paragraph, and there was a link in the next paragraph, when joining the two paragraphs, and going to the end of the line to continue writing, everything from the small font thing to the end of the line (new things i wrote) disappeared suddenly.

This prompted me to try and reproduce. With the steps above it doesn't delete the text if you write after that weird smaller text, but it did for me in that other ocasion.