SUMMARY
A file that is called
A complete guide to Linux'$'\n''process scheduling.pdf
in Plasma Dolphin is shown as
A complete guide to Linuxprocess scheduling.pdf

In other applications file picker windows, it is shown as
A complete guide to Linux
but if you pass the mouse pointer on the file, you get the following hint (yes it's made of two lines)
A complete guide to Linux
process scheduling.pdf

Here the steps I followed, that leaded me to this point
I found out that because nextcloud client
I was reading in Okular the file
https://tampub.uta.fi/bitstream/handle/10024/96864/GRADU-1428493916.pdf
which title on first page is
A complete guide to Linux process scheduling
when I selected and copied the title on first page in Okular, then selected "Save file" and pasted the title as file name.
Immediately I got an unknown error from Nextcloud client, so I started investigating why this file could not get synchronized. I noticed that there was a missing space between the words "Linux" and "process", and wondering if the copy paste from a PDF could have introduced weird chars, I runned
# ls /foo_dir
and found out that the real filename was
A complete guide to Linux'$'\n''process scheduling.pdf

According to 
https://en.wikipedia.org/wiki/Ext4
=======
Allowed characters in filenames	All bytes except NUL ('\0') and '/' and the special file names "." and ".." which are not forbidden but are always used for a respective special purpose.
=======
so '$'\n'' is a string for a filename, and I think it should be displayed