To be able to see the button text
On some themes, the button text defaults to being very dark
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馃憤 Twenty Nineteen is a good example of that.
If it's not going to be a big headache, adding a CSS class wp-block-button to the div.wp-block-jetpack-eventbrite.is-modal-button in the markup might be a quicker solution. Like...

That would make the pill shape button like other buttons in the template-first themes because they override the default button style with .wp-block-button:/ Ideally, this should be fixed in the themes, but I doubt that's going to happen as no one is actively working on the template-first themes at the moment. I think Calendly block does the same workaround. What do you think?
I think the change proposed by @iamtakashi is not a workaround but the intended behaviour of wp-block-button__link.
As far as I can see in Core Guten, wp-block-button__link is the inner element inside a wp-block-button.
So, it's to be expected that it malfunctions when used "outside" wp-block-button.
E.g. this is how a normal Button block is rendered in the editor:

Agreed, not adding the wp-block-button class was an oversight--that seems like the best solution to me because it emulates the Core button block (which was the idea in implementing to modal embed type for Eventbrite).
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I think the change proposed by @iamtakashi is not a workaround but the intended behaviour of
wp-block-button__link.As far as I can see in Core Guten,
wp-block-button__linkis the inner element inside awp-block-button.So, it's to be expected that it malfunctions when used "outside"
wp-block-button.E.g. this is how a normal Button block is rendered in the editor:
