Well, as I was trying to explain
here, sometimes replace all doesn't find occurences of said string on the open file, I mean the file open when I launch fhe global find/replace. I see it. I re-launch the find/replace and it doesn't find anything, even if I can see the string with my eyes. But when I switch and open another file (in the same ePub I mean), I repeat the exact same find/replace and it finds the occurences it didn't find a second ago.
I don't know how to replicate this, I will try to note what I did exactly next time it happens, but it doesn't seem like a very stable problem. And I seem to be the only one with the problem, so I'm more than willing to admit that I'm doing something wrong but I just can't see what.
But first, I will try anyway, as Serpentine suggested, to always put the cursor at the start of the file and see if the problem still occurs.