jq - Discrepancy in newline treatment between bash and zsh and zsh-subshell - jq

Question

I'm trying to format a string as JSON using jq and I noticed differing behaviors on bash vs zsh; specifically when zsh runs jq directly, the outcome is different than when it runs it in subshell: \n input gets output as \\n in first case, vs as\n in the latter.
I'm puzzled and not sure what's going on there:
Is this a known zsh behavior?
Is this a jq bug?
Or does it work as designed and I'm missing something?
BTW: Use newline with jq suggests to use printf %b to obtain \n instead of \\n, which works for bash,. but discrepancy in zsh between the modes is still there.
$ jq --version
jq-1.6
# ---
# Using \n directly
bash-3.2$ jq --null-input --compact-output --raw-output --monochrome-output --arg test 'A\nB' '{test: $test}'
{"test":"A\\nB"}
bash-3.2$ OUT=$(jq --null-input --compact-output --raw-output --monochrome-output --arg test 'A\nB' '{test: $test}'); echo $OUT
{"test":"A\\nB"}
zsh-5.8.1> jq --null-input --compact-output --raw-output --monochrome-output --arg test 'A\nB' '{test: $test}'
{"test":"A\\nB"}
zsh-5.8.1> OUT=$(jq --null-input --compact-output --raw-output --monochrome-output --arg test 'A\nB' '{test: $test}'); echo $OUT
{"test":"A\nB"}
# -----
# Using `printf %b` to convert `\n` to real newline
bash-3.2$ jq --null-input --compact-output --raw-output --monochrome-output --arg test "$(printf %b 'A\nB')" '{test: $test}'
{"test":"A\nB"}
bash-3.2$ OUT=$(jq --null-input --compact-output --raw-output --monochrome-output --arg test "$(printf %b 'A\nB')" '{test: $test}'); echo $OUT
{"test":"A\nB"}
zsh-5.8.1> jq --null-input --compact-output --raw-output --monochrome-output --arg test "$(printf %b 'A\nB')" '{test: $test}'
{"test":"A\nB"}
zsh-5.8.1> OUT=$(jq --null-input --compact-output --raw-output --monochrome-output --arg test "$(printf %b 'A\nB')" '{test: $test}'); echo $OUT
{"test":"A
B"}

Answer 1

printf behavior is the same between both shells, as are all the shell expansions related to your jq invocation -- but default echo behavior differs.
You can avoid this by switching from echo to printf.
% OUT=$(jq --null-input --compact-output --raw-output --monochrome-output --arg test "$(printf %b 'A\nB')" '{test: $test}')
% printf '%s\n' "$OUT"
{"test":"A\nB"}