pg_stat_user_tables.n_tup_ins empty for partitioned table

Hi all,
I've got a table named "root", partitioned on a date field into years
(e.g., "2018") and into months like "y2018m11" using range
partitioning on PostgreSQL 11.2.
Tuples are inserted into root with an INSERT...SELECT. I have
performed an UPDATE due to a new column inserted in the parent table
y2018.

Today, checking the statistics of the table, I found that no tuples
were inserted:

testdb=> SELECT relname, seq_scan, idx_scan,
n_tup_ins, n_tup_del, n_tup_upd, n_tup_hot_upd,
n_live_tup, n_dead_tup,
last_vacuum, last_autovacuum,
last_analyze, last_autoanalyze
FROM pg_stat_user_tables;

-[ RECORD 12 ]---+------------------------------
relname | y2018m11
seq_scan | 42172
idx_scan |
n_tup_ins | 0
n_tup_del | 0
n_tup_upd | 28191462
n_tup_hot_upd | 86
n_live_tup | 14086279
n_dead_tup | 0
last_vacuum |
last_autovacuum | 2019-03-28 17:23:35.909943+01
last_analyze |
last_autoanalyze | 2019-03-28 17:25:12.773707+01

I suspect the fact that n_tup_ins is 0 is due to the fact that I did
insert the tuples into the parent y2018, but I would like to better
understand: how can have n_live_tup without having n_tup_ins?
Moreover, I don't have any stat for the parent table root, neither for
y2018.
Some explaination/pointer is appreciated.

Thanks,
Luca