Hi Razvan,
You could create a custom class for it, for instance btn-nounderline which you place on the button.
in you scss file, add the following code:
.btn-nounderline {
a:hover {
text-decoration: none;
}
}
no sure how to add this using SASS? Here is is quick how-to detailing how to add custom classes: https://www.mxblog.nl/2016/03/using-sass-with-mendix/
Its old(-ish) but still valid
Good luck!