Last time i had this it was a kink in the rail, just required a bit of work straightening it out. Also seen it happen because part of the rail had shifted slightly, prying it back into place with a screwdriver fixed that one.
I would suggest that you take the trims off and wind the windows up and down several times to see where it shakes around or wobbles a bit before it falls out of the tracks. Thatll give you a good indication on what needs to be fixed.
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