Today is the first time I had to use the new UI, as I wanted to delay for a week my subscription delivery (but keeping the same frequency in terms of weeks). When doing so the subscription just doubled (that is the system kept the initial delivery for the original dates and added a second delivery a week later). I tried to undo this by changing the date back to the original one but then there was one delivery with twice as much of products, which also could not be reduced by editing the quantity through the system. I finally cancelled the subscription (that had a 10EUR discount code on, due to a 2 weeks delay on the previous order) because I could not see any other way to fix this, but I actually just wanted to delay a week the delivery.
I think you should check what is going on with the UI, and also I would eventually want to have my subscription back (with the 10EUR discount code)