Yeah, atstrings is a pretty awkward feature. It is possible to opt in and out of it, but only at the top level:
(declare 'atstrings t)
(declare 'atstrings nil)
This affects how strings are processed by 'ac. That is, it only matters for compilation/macroexpansion phases.
I see some of Anarki's tests set 'atstrings to a particular value as they go along, but then they don't reset it. This might be why you see some tests working when yours don't.
(In Anarki, the declarations* global table holds these values, so it's technically possible to save and restore the old value by reading that table.)
The REPL behavior you observed is confusing in a different way than you thought. It treated your input as a malformed command, three well-formed commands you didn't know you were entering, and finally an unfinished string literal:
This is where the "\n" response came from, and it's why you received multiple responses per input line.