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: 7.5.3.1 (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 Calc: threaded
I see this in 7.5 too. So, WFM I guess.