Leaflet is using square BBOX parameters for individual tiles. Sentinel Hub service allows any geometry to be passed for clipping.
I am not sure, what your use-case it, but unless you really want to have the rectangle cropped, you can simply delete these bounds. If you want to crop per field boundaries, you will have to integrate this in your Leaflet app. See information on how to do that with the service here:
https://www.sentinel-hub.com/faq/how-can-i-clip-image-specific-polygon