I came across this issue as I wanted to extend the PATH variable for all users by adding another directory at the end of PATH in /etc/environment.
Neither opening a new shell nor a fresh login did show the extended PATH.
I found that the protections on /etc/environment were set to
-rw-r--r-- 1 root root 588 May 13 12:08 /etc/environment
Setting /etc/environment to be executable fixed this for me.