Selecting a theme gradient doesn't update the gradient bar color

[mirka]

Clicking on a theme gradient doesn't update the gradient bar UI:

The isolated instances in Storybook for the GradientPicker component are working fine, so there is likely something wrong with the data flow in the block editor usage.

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Reported by @jasmussen

[luminuu]

Upon searching for this behaviour I found this issue and can confirm it does happen when CSS variables are used. Twenty Twenty-Four uses hex codes in the gradients, while for example Twenty Twenty-Three in the Sherbet style variation, CSS variables are used. It is reproduceable there.

To reproduce, the use code like this in a gradient in theme.json:
linear-gradient(135deg,var(--wp--preset--color--primary) 0%,var(--wp--preset--color--secondary) 100%)