GROUP BY on view queries
You are right that table_a1_column is functionally dependent on the other columns, but PostgreSQL isn't smart enough to deduce that.

If view_a were a regular table, PostgreSQL would know that the columns in the GROUP BY clause contain the primary key, but since it is hidden behind the query rewrite rule of a view, it cannot infer that.

For PostgreSQL there are no functional dependencies between the columns of a view. It might be technically possible to improve that, but it is doubtful whether that would be worth the extra effort during query planning, particularly since the query can be fixed by simply adding all columns to the GROUP BY clause.