Hi there!
The icon disappear issue will be solved with the following code I hope:
.sp-megamenu-parent .sp-dropdown .sp-dropdown-items .sp-has-child>a:after {
font-family: 'Font Awesome 5 Free';
font-weight: 600;
}
Please put this in your custom CSS of the Template Options.
Note: Path towards custom CSS(Joomla 3): Extensions>Templates>Styles> Click on your current default template name then choose Template Options. Then find custom CSS within the custom code.
However, if you have updated the system plugin of helix too, then it will be better to roll back to your last working backed up version of the site. The Qubic template does't get the compatibility update for new helix3 framework, and it may cause several inconsistancies as you are facing now.
Hope this helps!
Best Regards