Apparently,

recent versions of ssh started using pipes for non-TTY STDIO

so

there is no way for a bash script to tell whether a non-tty ssh command is being piped or not.

Since nohup only springs into action when it detects that stdout/stderr are connected to a terminal (and does nothing when the output goes to a pipe), and ssh's way of launching it looks like it's being piped, you get the behaviour you're seeing.

As muru points out, you can force ssh to allocate a tty by using the -t parameter:

$ ssh -t localhost "/usr/bin/nohup /bin/echo foo"
/usr/bin/nohup: ignoring input and appending output to ‘nohup.out’

Apparently,

recent versions of ssh started using pipes for non-TTY STDIO

so

there is no way for a bash script to tell whether a non-tty ssh command is being piped or not.

Since nohup only springs into action when it detects that stdout/stderr are connected to a terminal (and does nothing when the output goes to a pipe), and ssh's way of launching it looks like it's being piped, you get the behaviour you're seeing.

As muru points out, you can force ssh to allocate a tty by using the -t parameter:

$ ssh -t localhost "/usr/bin/nohup /bin/echo foo"
/usr/bin/nohup: ignoring input and appending output to ‘nohup.out’

Apparently,

recent versions of ssh started using pipes for non-TTY STDIO

so

there is no way for a bash script to tell whether a non-tty ssh command is being piped or not.

Since nohup only springs into action when it detects that stdout/stderr are connected to a terminal (and does nothing when the output goes to a pipe), and ssh's way of launching it looks like it's being piped, you get the behaviour you're seeing.

As muru points out, you can force ssh to allocate a tty by using the -t parameter:

$ ssh -t localhost "/usr/bin/nohup /bin/echo foo"
/usr/bin/nohup: ignoring input and appending output to ‘nohup.out’

Apparently,

recent versions of ssh started using pipes for non-TTY STDIO

so

there is no way for a bash script to tell whether a non-tty ssh command is being piped or not.

Since nohup only springs into action when it detects that stdout/stderr are connected to a terminal (and does nothing when the output goes to a pipe), and ssh's way of launching it looks like it's being piped, you get the behaviour you're seeing.

As muru points out, you can force ssh to allocate a tty by using the -t parameter:

$ ssh -t localhost "/usr/bin/nohup /bin/echo foo"
/usr/bin/nohup: ignoring input and appending output to ‘nohup.out’