What do null column values for pg_stat_progress_vacuum mean?

Hi,

We have an autovacuum process that has been running for almost 27 hours:

SELECT * FROM pg_stat_activity WHERE pid = 11731;
-[ RECORD 1 ]----+---------------------------------------
datid            | 16385
datname          | database_name
pid              | 11731
usesysid         |
usename          |
application_name |
client_addr      |
client_hostname  |
client_port      |
backend_start    | 2020-03-04 23:40:14.828138+00
xact_start       | 2020-03-04 23:40:14.849367+00
query_start      | 2020-03-04 23:40:14.849367+00
state_change     | 2020-03-04 23:40:14.849368+00
wait_event_type  |
wait_event       |
state            | active
backend_xid      |
backend_xmin     | 3801997676
query            | autovacuum: VACUUM public.responses
backend_type     | autovacuum worker

A row shows up in pg_stat_progress_vacuum, but it contains null values
for every column.

SELECT * FROM pg_stat_progress_vacuum WHERE pid = 11731;
-[ RECORD 1 ]------+---------------
pid                | 11731
datid              | 16385
datname            | d2j496215lfs41
relid              |
phase              |
heap_blks_total    |
heap_blks_scanned  |
heap_blks_vacuumed |
index_vacuum_count |
max_dead_tuples    |
num_dead_tuples    |

I see nothing in the documentation to suggest that this is an expected
state - what does it mean?

Thanks,
Mark.