I don't think that can happen, because the mask is always [explicitly set](https://github.com/rhattersley/iris/blob/bilinear-new-modes/lib/iris/experimental/regrid.py#L264). (The `data` variable name is [re-used elsewhere](https://github.com/rhattersley/iris/blob/bilinear-new-modes/lib/iris/experimental/regrid.py#L331) which probably obfuscates this!)