Referring to this old thread

I think what it probably is, is that it is it not, not remembering them it's that the find in files combo is also populated by Find all occurrences and also a simple Ctrl+F find within this file

So if you do a find in files and then do more than ten of the other two in any combination then when you return to your find in files the last thing you searched on has fallen off the bottom of the list