Hi there!
Thanks for your query.
It seems caused by extra CSS code. Would you please give me your admin access to check?
Or, you can simply use the following code in your custom CSS:
@media (max-width: 991px){
.nav-placeholder, #sp-header {
display: block !important;
}
}
Note: Path towards custom CSS(Joomla 4): System>Site Templates Styles> Click on your current default template name then choose Template Options. Then find custom CSS within the custom code.
https://prnt.sc/5mBuSRh907N3
https://prnt.sc/ooiq-lkMEXr_
https://prnt.sc/nunlX-f5AhK1
Best Regards