I can’t name the precise problems but I will say that trying to grow mint indoors is always going to be fraught. Your plant is etiolated and the consequent weak sappy growth will be prey to any passing bug. Can you grow it outdoors where it will have sufficient light and where pests will be kept in bounds by the weather?

@floral_uk z.8/9 SW UK It gets full sun in the windowsill, but is that enough? The only has outdoor place I have access to is a darker place without much full sun.

I would still put it outside, after hardening it off. Our eyes are not good at ascertaining true light levels and it’s very possible that what appears to be a ‘dark’ spot outdoors is still brighter than the indoor position. Your mint definitely needs more light as can be seen by the long internodes. Also, outside it will be subject to wind and rain which will literally toughen it up and make it less susceptible to problems,