Free people one shoulder side split polka dot maxi dress size 12 - runs small

Adorable polkadot maxi dress from free people. Has a side zip and one shoulder and a slit that comes just above the knee on my 5 foot seven frame. This has a hidden zip on the side with no shoulder. This is a US size 12 UK size 16, but it definitely feels like it runs small, and there is no stretch. It’s more like a 10. Measures 19” across P2P.