Is the field that contains the word of type text or type number? You can enter text into number fields, but since the indexing is different, this affects how searches and sorts based on this field behave.
It's also possible that the index of the field is corrupted. You can turn indexing off, exit manage database, then return and turn it back on to rebuild the index of a given field. You can also rebuild all the indexes of a file by importing all the data into a clone of the file.
Once I've confirmed that the field in question is either a text field or a calculation that returns text, I'd see if rebuilding the index corrects the problem. I'd also probably run a recover on the file to see if any other problems may exist with the file.
Best practice is to never use a recovered file going forward. Instead, import the data from the recovered file into a clone of an undamaged back up copy if at all possible.
TX for your response ! The field is of type text in which you can only import data from a value list. I tried turning indexing off and back on, but it didn't make any difference. I find it very peculiar that it concerns only this one particular word (I've tested all the others and it works).
What is the word?
Are you performing the find manually or in a script?