I expect what’s in the string to begin with is not what you think is in the string to begin with.
In Clarion the Single < has specific meaning when used in a String that converts to Binary ASCII codes.
So
st.Replace('<0dh,0ah>')
Means the same as
st.Replace( chr(0Dh) & chr(0Ah) )
But I’m not sure that your original text contains chr(13) chr(10). I think it contains the characters: <0Dh> etc. As evidenced by you seeing that string in the Debugview output.
(it’s weird to see text like that in a string, but hey, it’s your string.)
You might have more success with;
st.Replace('<<0dh,0ah>')
That forces the first parameter there to be “plain text”.