yeah I reported this bug 2 years ago but it was never fixed.
A workaround would be to concatenate the JSON into the formula body string you’re passing to fl(). tc(utf, 22) returns a doublequote ".
$fl(1, 6, "i+1", "tc(json, " + tc(utf, 22) + #v + tc(utf, 22) + ", i)", ",")$
You can also use a placeholder and replace it with a doublequote like this:
$fl(1, 6, "i+1", tc(reg, "tc(json, '" + #v + "', i)", ', tc(utf, 22)), ",")$