If I use the UPDATE GROUP method to rename an M365 group, it does not update the title of the related SharePoint site. I have read a few posts about that the modification takes effect with a delay, but the site still uses the old name even weeks later. This behavior also affects other products, like Onedrive. The only workaround I have found is that I have to open the SPO site with group owner privileges and open the group settings panel from the homepage. In that case, the site gets renamed immediately.