Adding column last / first

When the column wsa added last, DML is immediately available.
However, when the column was added first, it take about 1 minute for DML.
what mechanism is different in two case?

alter table TEST add column C20 int;
select sum(C20) from TEST_01;

alter table TEST add column C20 int first;
select sum(C20) from TEST_01;


ALTERs only block concurrent DML queries during the metadata switch over phase of the ALTER (a few seconds), not while moving data (here is a blog post about the process ALTERs do need to wait until no queries are running to do the metadata switch, so an ALTER during a long running query can cause some extra blocking as its waits on that query to complete.