Bootstrap icons seem no longer to be usable in AUI Buttons as stated in documentation: https://docs.liferay.com/portal/7.0/taglibs/util-taglib/aui/button.html
While there were no issues in 7.2.x the icons simply don't show up in 7.3.x
Steps to reproduce:
Create a MVC portlet with to following content in a jsp file:
A button with a pencil appears.
A small button with no icon appears.