Hi There!
Sorry for your issue.
The Spectrum Template is based on the Helix3 framework and this framework got a recent update to make it compatible with Joomla 4 and so it has adopted a lot of changes and updated versions of core components, such as Bootstrap, fontawesome, and so on.
All of our Helix3 based templates will get updates soon too. Kindly, do not update the system plugin until there is an update for the template itself. It will be better to roll back to your previous working backed-up version.
However, the reported issue can be solved with the following custom CSS code though:
#sp-header .row {
display: flex;
}
Note: Path towards custom CSS: Extensions>Templates>Styles> Click on your current default template name then choose Template Options. Then find custom CSS within the custom code.
Best Regards