impossible to save source code in wysiwyg blocks if  does not contains some strings
If this only happens when a Text format is associated with a WYSIWYG profile, regardless of the format's Text filters or the Editor assigned to it, then try using a non-breaking space (&amp;nbsp;) as the anchor text:

&lt;a href="https://myfburl"&gt;&lt;i class="fab fa-facebook-f "&gt;&lt;/i&gt;&amp;nbsp;&lt;/a&gt;