Wait who put the curse on the uke in the first place? I feel like i lost the plot because ...

it was a joke i think

Oh like an ending joke! I see i see

It’s a nonsensical plot but in ch 2, there’s a flashback of Yui typing a wish on a witch counselling website he found. He didn’t read the Terms and Condition which said his body will be changed to grant his wish and this ending shows the Terms and Condition also had a clause that the curse will be broken when he finds true love.