Text swatch is rendered as image – why?

Magento 2.4.6

I have two product attributes, both are text swatches used on a product.

One of them is rendered as an image with the class swatch-option image
while the other is rendered as swatch-option text.

I just can’t figure out why one is rendered as an image … they look the same and are definitely text swatches. Is there a way to force them not to be rendered as image? What could be the reason for that behaviour?

Thanks!