Hi there,
its because that button was copied from your post content to the footer at some point.
when you copy a block from one post to another ( including GP Elements ) it does not update the unique class on the block.
So that footer button has the same class as button on your homepage which is overriding it styles.
To fix it.
Edit the site footer. Select the BUTTONS ( wrapper ) block and duplicate it:

Then delete the original buttons block. The duplicate will generate a new class.