I think that this sentence is not true, when you assigns a default value in the attribute, the value is populated in odf_ca_project table
I perfomed the following
1 - create custom attribute nicolas (data type = string) / default value = hello
2 - add the attribute in both views (create / edit)
3 - enable audit trail with the above attribute
4 - go to Project / create new Project / save
I checked odf_ca_project according my new Project id = 5****** and the value in my custom attribute (by default) appears in the column,
Now, I changed the value from 'hello' to 'how' and audit trail change new value (hello) by new value (how), a correct a pattern because before there was nothing
Best Regards,
Nico!