Flashback Data Archive and FBAR Timer wait event on alter table

Hello

As part of testing FDA on our 19.18 database we created this setup

SQL> create flashback archive FBA tablespace FDA_DATA quota 5G retention 10 day;

SQL> GRANT FLASHBACK ARCHIVE ON FBA to TESTUSER;

SQL> alter table TESTUSER.TEST_TABLE FLASHBACK ARCHIVE fba;

Everything works fine

Now we tried to alter that table to add a column

alter table test_table add (DESCRIPTION2 varchar2(53));

It just sits there and nothing is running

What is going on here? How can we address this? How do we alter a table that is part of the flashback archive?

Thank you in advance for your guidance and help