Non-English filename disappears after coppying.

Maybe the target filesystem doesn't support those filenames. I think I saw that either with NTFS or SAMBA. Really annoying.

Is that a limitation of the destination filesystem?
- There's an Arabic name in the bottom right. Likely the software getting confused.
  
  The filename ending left to right despite the file being right to left is kinda funny

Can you check in a terminal? If you can see them in the terminal and not in the desktop you're missing a font. If you can't see them in the terminal then you've somehow mangled them. What was the OS and filesystems you copied from?
- Unless the font if the terminal has the same issue
  
  Redirecting the output of ls to commands such as hexdump or od would allow to notice if the name has international characters or if they were replaced by some placeholder character (which would be represented as a repeated value across the hex dump)

Are the missing ones Arabic, too? If so, there is a "noto sans Arabic" font and other symbol fonts that you'll need to have installed. That's really weird, though. I have Arabic everywhere and I don't have this issue. Did you uninstall any fonts by any chance?
- Yes, those are Arabic, but Chinese, Japanese and Bangla are also affected. And I have the fonts installed.
  
  I'd try reinstalling the plasma workspace and see if that fixes it. Or even the whole desktop.
  For some reason, I assumed you have plasma installed

Could be a bug in Nautilus though it's so mature now that would be strange. I'd report it to their repo (don't have the link and I'm on my phone but it should be easy to find).
ext4 supports various filename encodings (simultaneously, even!) but sometimes when you copy a file from one destination to another in a batch with mixed encodings you can end up with situations like this. Especially from within a GUI.
Does the problem occur when you copy each file one by one or only in batch?
- This happened when copying in batch.