Does setting a default value for Alter Table in Postgres make the query significantly slower?

We had an incident very recently that caused our service to be unavailable for some minutes. Here is the liquibase script that led to the incident:

As you can see, the changeset took quite a lot of time. Our team has concluded that the default value set to null is the one to blame here. However, it somehow doesn't sit right with me. When I check the Postgres pages 1 and 2 , I see that setting the values to null is no different than removing the default value altogether. So, I don't see why this default value setting would cause such problems. On a side note, there were 12 million rows in the table. As in the title, my question is, does setting a default value cause such downtimes? The application startup:

Started ApplicationKt in 289.989 seconds (process running for 290.727)

Some logs before the liquibase operation: