Did you ask the luthier specifically about it?
Make sure the string is deep enough down, and placed so that it will be tightened (for example if there is a mark from the string that it easily can fall into, and get out of, don't put it there), and tighten it until the screw is completely stuck. Play for a bit (without bending), and in an hour or so, check the screw again. You can see if the problem is the screw untightening itself, or if the string will somehow manage to pull itself out. Unless of course, the problem happents immediately after putting on new strings.
Does it happen to all the strings, or just some specific ones?
How does the blocks that tighten it look?
Hope you solve it, good luck
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