remote ssh command: first echo output is lost - linux

Question

I'm trying to run several commands on a remote box via ssh 1-liner call by specifying them as semicolon-separated string passed to "bash -c". It works for some cases, but does not for others. Check this out:
# Note: the "echo 1" output is lost:
bash-3.2$ ssh sandbox bash -c "echo 1; echo 2; echo 3"
2
3
# Note: first echo is ignored again
bash-3.2$ ssh sandbox bash -c "echo 0; echo 1; echo 2; echo 3"
1
2
3
# But when we run other commands (for example "date") then nothing is lost
bash-3.2$ ssh sandbox bash -c "date; date;"
Wed Nov 7 20:27:55 UTC 3018
Wed Nov 7 20:27:55 UTC 3018
What am I missing?
Remote OS: Ubuntu 16.04.5 LTS
Remote ssh: OpenSSH_7.2p2 Ubuntu-4ubuntu2.4, OpenSSL 1.0.2g 1 Mar 2016
Local OS: macOS High Sierra Versoin 10.13.3
Local ssh: OpenSSH_7.6p1, LibreSSL 2.6.2
Update:
The above example is heavily simplified picture of what I'm trying to do.
The practical application is actually to generate few files on remote box by echo'ing into remote filesystem:
#!/bin/bash
A=a
B=b
C=c
ssh -i ~/.ssh/${REMOTE_FQDN}.pem ${REMOTE_FQDN} sudo bash -c \
"echo $A > /tmp/_a; echo $B > /tmp/_b; echo $C > /tmp/_c;"
After I run the above script and go to remote box to check results I see the following:
root#sandbox:/tmp# for i in `find ./ -name '_*'|sort`; do echo "----- ${i} ----"; cat $i; done
----- ./_a ----
----- ./_b ----
b
----- ./_c ----
c
As you can see the 1st "echo" command generated blank file!

Answer 1

To be clear, there's 3 shells at work here - the one that interprets ssh, your local shell that is; the one that ssh will be automatically running for you, and the bash you're invoking explicitly.
The reason the 1 is "disappearing" is that the shell that interprets the ssh command "eats" the quotes around the -c arguments, and then the shell on the other side of ssh splits the arguments at whitespace. So it ends up looking like bash -c echo 1 ; echo 2; echo 3. In turn, -c just gets echo, which echos an empty line; 1 becomes the value of that shell's $1, which isn't used. Then the inner bash returns, and the direct ssh shell runs the echo 2; echo 3 normally.
Consider this:
$ ssh xxx bash -c "'echo 1'; echo 2; echo 3"
1
2
3
where echo 1 is protected within the ssh arguments, so the 2nd level ssh shell is passed bash -c 'echo 1'; echo 2; echo 3. The innermost 3rd level shell echos 1, and then the 2nd level ssh shell echos 2 and 3.
Here is yet another interesting permutation:
$ ssh xxx bash -c "'echo 1; echo 2; echo 3'"
1
2
3
here, the inner shell gets all the echos as they're kept grouped within the first shell by " and within the second shell by '.
In general, shell scripts to pass arguments to shell scripts that run shell scripts can be pretty difficult to build. I'd recommend you change your technique a bit to save yourself a lot of effort. Instead of passing the shell commands as command line parameters to the ssh argument, instead provide it through the standard input to the shell. Consider using a pipeline like this, which avoids recursive shell interpretation:
$ echo "echo 1; echo 2; echo 3" | ssh -T xxx
1
2
3
( Here, the -T is just to supress ssh complaining of lack of pseudoterminal).

Answer 2

All the arguments to ssh are combined into a single whitespace-separate string passed to sh -c on the remote end. This means that
ssh sandbox bash -c "echo 1; echo 2; echo 3"
results in the execution of
sh -c 'bash -c echo 1; echo 2; echo 3'
Note the loss of quotes; ssh got the three arguments bash, -c, and echo 1; echo 2; echo 3 after quote removal. On the remote end, bash -c echo 1 just executes echo, with $0 in the shell set to 1.
The command
ssh sandbox bash -c "date; date;"
is treated the same way, but now the first command contains no whitespace. The result on the remote end is
sh -c 'bash -c date; date;'
which means first a new instance of bash runs the date command, followed by the date command being executed directly by sh.
In general, it's a bad idea to use ssh's implicit concatenation. Always pass the command you want executed as a properly escaped single argument:
ssh sandbox 'bash -c "echo 1; echo 2; echo 3"'