In theme_t3kit/Resources/Private/Templates/ContentElements/IconTextButton.html, the fluid-variable anchorClass has a faulty logic (imho).
Only if settings.disableWholeAreaLink is 1 AND settings.btnAsLink is 0 should the classes "btn btn-default" be set (and added to icon-text-btn__link).
More simply put : If the "whole area link" is disabled and "button as link" is disabled, then apply the button-classes.
This is how it currently is:
<f:variable name="anchorClass">{f:if(condition: '{settings.disableWholeAreaLink} != 1 && {settings.btnAsLink} != 1' , then: '', else: ' btn btn-default')}</f:variable>
And this is what I suggest:
<f:variable name="anchorClass">{f:if(condition: '{settings.disableWholeAreaLink} == 1 && {settings.btnAsLink} != 1' , then: ' btn btn-default', else: '')}</f:variable>