@MarkI have tried with same configurations as you mentioned in the question in extract text and in replace text processors, The output flowfile contents from replace text processor resulting with \ in it.
Output:-
Here is my configurations:-
Extract Text:-
Output from extract text processor:-
abc attribute value having \ in it
Replace Text:-
As Regex Replace as Replacement Strategy
(Or)
Replacement Strategy as Always Replace
Output:-
Both of these Replacement strategies are resulting same output that i mentioned as first screenshot.
Once check the attribute abc value after Extract Text processor having \ in it or not.
If you still having issue then share your flow and configs of the processors please..