I never had this kind of problem - and I don't know why..
some icons are missing and appearing blinking with "?/!"
what is happening?
Its a submit button. I have the same icon in another button - and it's ok there.
If you are using the newest version of Font-Awesome,
fa is no longer a valid class. It has been replaced with
fas (solid), and
fab (brands). For a thumbs up, you would use:
<i class="fas fa-thumbs-up"></i>
Here is the documentation if you want to learn more.