Alert don't return the source user

Alert don't return the source user

Hi @all,
Since some times (i don't know how much), when someone from my network modify the default domain policy GPO, i get this message :
GPO Default Domain Policy was modified by at 11/10/2019 11:06:29.
Which is great but the username is missing after "by".
What should i check to resolve this issue ?
Thanks a lot.
Regards,