@Kacper Bury Hello and welcome to Microsoft Q&A
I understand you are getting unexpected results when using DataFlow to write to SFTP. Namely getting multiple files instead of a single.
From what you have written, I see nothing wrong. I can think of two things.
Did you do this in "debug run" or "trigger now"/triggered run? I'm thinking maybe it ran an old version (published) of the pipeline instead of the one with intended settings (debug).
The other thing to try, is setting file name options: output to single file
Ideally, it should have only writen one data file with single partition. Was the other file named like _success ?