Yes potassium deficiency, but not due to lack of potassium in the media
The thing is potassium is often not the issue, the problem isn't availability of K in the media.
simply in this case
you need to dry your growing medium out between each watering, (small signs of mag/cal deficiency just starting) dry your medium out between watering.
dry your pots out between waters, this fluctuates PH and thus nutrient availability
very minor at this stage and not much to concern yourself with, your plants look fine.