partition pruning only works for select but update

Hi,
We have a table have range partition (about 5K partitions) , when
Explain select count(*) from table where partitionkey between to_timestamp() and to_timestamp();
It show
Aggregate (cost=15594.72..15594.73 rows=1 width=8)
-> Append (cost=0.15..15582.00 rows=5088 width=0)
Subplans Removed: 5086

But when
Explain update table set .. where partitionkey between to_timestamp() and to_timestamp();
It still show all of partitions with update ...

enable_partition_pruning keep defaut value 'on', It's expected ? and we found for update sql with same where condition, it consumes huge memory than select.
Database version is Postgres 13.4 on RHEL8.4.

Thanks,

James