Conversion of plain-text files from ASCII to Unicode without any command

Why are ASCII-encoded files extended to UTF-8 or in reverse reduced to ASCII?

user:~$ echo 'A  B  C  |  }  ~' > ./file 
user:~$ 
user:~$ file --brief --mime ./file
text/plain; charset=us-ascii
user:~$ 
user:~$ 
user:~$ echo 'ᴁ  ♫  ⼌  𝐑  🀵  🈀' >> ./file 
user:~$ 
user:~$ file --brief --mime ./file 
text/plain; charset=utf-8
user:~$
user:~$  
user:~$ cat ./file 
A  B  C  |  }  ~
ᴁ  ♫  ⼌  𝐑  🀵  🈀
user:~$ 
user:~$ 
user:~$ sed -i '$d' ./file 
user:~$ 
user:~$ cat ./file 
A  B  C  |  }  ~
user:~$
user:~$ file --brief --mime ./file 
text/plain; charset=us-ascii
user:~$

In case you cannot read a character in the second echo statement: From first to last: U+1D01, ᴁ; U+266B, ♫; U+2F0C, ⼌; U+1D411, 𝐑; U+1F035, 🀵; U+1F200, 🈀.

The locale settings are:

user:~$ echo $LANG
en_US.UTF-8
user:~$ echo $LANGUAGE
en_US:en
user:~$ echo $LC_COLLATE

user:~$ echo $LC_CTYPE

user:~$ echo $SHELL
/bin/bash
user:~$ echo $SHELL
/bin/bash
user:~$ 
user:~$ ps -p $$
  PID TTY          TIME CMD
 7537 pts/6    00:00:00 bash
user:~$