My find/replace text magically changes in Xcode
Find/replace uses what's known as the Kill Ring, rather than regular Copy/Paste

It's similar to the regular Copy/Paste buffers, but uses a separate memory area.  

You can 'Kill' using  Ctrl ⌃   K  which is similar to Cut &amp;
'Yank' using  Ctrl ⌃   Y  which is like Paste.

The kill ring is also used in a lot of search routines, so will be replaced any time you perform a new search.
Don't rely on it as a data store.