What happens in DXP 2026.Q1.8:
On first deployment of the Site Initializer CX and setup of the new Site, the friendlyURL is set to /{siteName}
After all subsequent deployment, friendlyURL is always set to /group-{groupId}. When it is manually changed in the Admin UI and the Site Initializer CX is redeployed or Liferay is restarted it’s set back to this value.
The docs say this about the site-configuration.json file:
You can check the values for these configurations by inspecting data from the
Group_database table in an existing site.
But as seen in the source here, it only handles a few of the attributes and ignores the rest, like “friendlyURL” 
It’s still a bug that the friendlyURL is changed after the first deployment of the CX.