When, in an LO module, you go to File | Templates | Manage - you see a small icon next to the default template indicating that it's the default. Why should this not be indicated when examining templates in the Start Center?
After making a template the default in File > Templates > Manage, I can see the same icon in the Start Centre. Looks like it was added in 7.5.
What is your version information?
Version: 126.96.36.199 (X86_64) / LibreOffice Community
Build ID: d29ee673721b12c92b3de9b9663473211414f0db
CPU threads: 8; OS: Linux 5.15; UI render: default; VCL: gtk3
Locale: en-AU (en_AU.UTF-8); UI: en-US
I see this in 7.5 too. So, WFM I guess.