When ContainerType of SfTextInputLayout is set to 'Outlined', it seems that hint label inherits its background color from the element on which SfTextInputLayout is actually placed to (see image).
In the image, it is visible that hint label background color is white, which actually would make sense in case that label doesn't overlap control's frame. But in any other scenario, it would be much better to be pink or even transparent.
Attachment:
Screenshot_20210110095030_c0c5cc3e.zip