I'm pretty sure its nothing to worry about can happen. But your plant would be much better in the ground - that pot will soon be much too small.

Maybe a nutrient deficiency?

The pot is already far too small for your rhubarb. Let it die back and in autumn plant the crown in well prepared soil with lots of rich organic material - compost or well rotted manure. Next year simply leave it to grown and do not pull any of the stalks it will take a few years to settle in but, once it does, you'll have plenty of rhubarb!