While testing some availability plugins, in particular availability_gwpayments, I noticed the float: right; CSS rule in the the Boost theme targets all <button> elements that are in .showmore-container.

This is the rule I'm referring to: https://github.com/moodle/moodle/blob/main/theme/boost/scss/moodle/core.scss#L3108-L3109

Here is a screenshot of a course page with Boost theme.
Notice how the "Select payment type" button is floating to the right. Although it is part of the first condition, the payment button looks like it belongs to the other 2 conditions, causing a false impression.

When switching to Classic theme, the "Select payment type" button correctly displays aligned to the middle, as part of the first condition.