@@Tryhard, @slowmo this looks like a transient magnesium issue. The immediate advice is to back off on nutrient strength as tryhard suggests and apply Calmag as a foliar to help correct the issue. The exact cause of the issue doesn't appear obvious with the info you have given as that all seems OK, but its probably down to environment in some way. Make sure you are getting run off from your pots to avoid build up in the media and try and keep your temps and humidity in optimum range so that transpiration stays normal.
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