scans on table fail to be excluded by partition bounds

[Justin Pryzby <pryzby@xxxxxxxxxxxxx>]

I came across a poorly performing report with a subplan like this:

ts=# explain SELECT * FROM eric_enodeb_cell_metrics WHERE start_time BETWEEN '2019-01-01 04:00' AND '2019-01-01 05:00' OR start_time BETWEEN '2019-01-02 04:00' AND '2019-01-02 05:00';
 Append  (cost=36.04..39668.56 rows=12817 width=2730)
   ->  Bitmap Heap Scan on eric_enodeb_cell_20190101  (cost=36.04..19504.14 rows=6398 width=2730)
         Recheck Cond: (((start_time >= '2019-01-01 04:00:00-05'::timestamp with time zone) AND (start_time <= '2019-01-01 05:00:00-05'::timestamp with time zone)) OR ((start_time >= '2019-01-02 04:00:00-05'::timestamp with time zone) AND (start_time <= '2019-01-02 05:00:00-05'::timestamp with time zone)))
         ->  BitmapOr  (cost=36.04..36.04 rows=6723 width=0)
               ->  Bitmap Index Scan on eric_enodeb_cell_20190101_idx  (cost=0.00..16.81 rows=6465 width=0)
                     Index Cond: ((start_time >= '2019-01-01 04:00:00-05'::timestamp with time zone) AND (start_time <= '2019-01-01 05:00:00-05'::timestamp with time zone))
               ->  Bitmap Index Scan on eric_enodeb_cell_20190101_idx  (cost=0.00..16.03 rows=259 width=0)
                     Index Cond: ((start_time >= '2019-01-02 04:00:00-05'::timestamp with time zone) AND (start_time <= '2019-01-02 05:00:00-05'::timestamp with time zone))
   ->  Bitmap Heap Scan on eric_enodeb_cell_20190102  (cost=36.08..20100.34 rows=6419 width=2730)
         Recheck Cond: (((start_time >= '2019-01-01 04:00:00-05'::timestamp with time zone) AND (start_time <= '2019-01-01 05:00:00-05'::timestamp with time zone)) OR ((start_time >= '2019-01-02 04:00:00-05'::timestamp with time zone) AND (start_time <= '2019-01-02 05:00:00-05'::timestamp with time zone)))
         ->  BitmapOr  (cost=36.08..36.08 rows=6982 width=0)
               ->  Bitmap Index Scan on eric_enodeb_cell_20190102_idx  (cost=0.00..16.03 rows=259 width=0)
                     Index Cond: ((start_time >= '2019-01-01 04:00:00-05'::timestamp with time zone) AND (start_time <= '2019-01-01 05:00:00-05'::timestamp with time zone))
               ->  Bitmap Index Scan on eric_enodeb_cell_20190102_idx  (cost=0.00..16.84 rows=6723 width=0)
                     Index Cond: ((start_time >= '2019-01-02 04:00:00-05'::timestamp with time zone) AND (start_time <= '2019-01-02 05:00:00-05'::timestamp with time zone))

Is there some reason why the partition constraints aren't excluding any of the
index scans ?  In the actual problem case, there's a longer list of "OR"
conditions and it's even worse.

The partitions looks like this:

Partition of: eric_enodeb_cell_metrics FOR VALUES FROM ('2019-01-02 00:00:00-05') TO ('2019-01-03 00:00:00-05')
Indexes:
    "eric_enodeb_cell_20190102_idx" brin (start_time) WITH (autosummarize='true'), tablespace "oldindex"
    "eric_enodeb_cell_20190102_site_idx" btree (site_id) WITH (fillfactor='100'), tablespace "oldindex"
Check constraints:
    "eric_enodeb_cell_20190102_start_time_check" CHECK (start_time >= '2019-01-02 00:00:00-05'::timestamp with time zone AND start_time < '2019-01-03 00:00:00-05'::timestamp with time zone)
Tablespace: "zfs"

And:
pg_get_partition_constraintdef | ((start_time IS NOT NULL) AND (start_time >= '2019-01-02 00:00:00-05'::timestamp with time zone) AND (start_time < '2019-01-03 00:00:00-05'::timestamp with time zone))

ts=# SELECT version();
version | PostgreSQL 11.4 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.4.7 20120313 (Red Hat 4.4.7-23), 64-bit

ts=# SHOW constraint_exclusion ;
constraint_exclusion | partition

ts=# SHOW enable_partition_pruning;
enable_partition_pruning | on

Thanks in advance.

Justin