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.