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Yup, it does looks like first sgin of magnesium deficiency.
https://loudclouds.co/wp-content/uploads/2016/07/marijuana-deficiency-chart-jorge-cervantes.jpg
Yep, I agree, magnesium deficiency.
Top dressing your pots with a small handful of souble magnesium sulphate powder should cure this.
Generally available from any good garden centre.
Just water it in after application, simple.