I would also like to know what causes it. Sometimes I think it could be sun damage, other times a sign of stress but I've seen it on a perfectly healthy good flowering phal too.
The only logical one left would be too cold nights which mine certainly get so could be that.
One thing I noticed is that the third picture looks like one I just discovered was getting way too little light in the spot it was in at mine. The new leaf has grown very narrow so if that is the case with yours and it has a lighter leaf edge that would rule out too much sun for that one.
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