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I've had this happen a couple of times recently in different projects.
I stuck a Matchbox effect (Vignetting most recently, but DropShadow before) on a bunch of clips in a bunch of sequences. All was well.
Later when I re-opened the project, all those clips were "Preview Unavailable" and the "MX-Vignetting" effect on the clips turned back into generic "Matchbox" with no shader selected.
In both cases these were the supplied .mx shaders; I will have to play around with uncompiled .glsl ones and see if it makes any difference.
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