BUG #16327: Too many files in pg_replslot folder

The following bug has been logged on the website:

Bug reference:      16327
Logged by:          Dmitry Romanenko
Email address:      romdn@inbox.ru
PostgreSQL version: 12.2
Operating system:   Windows
Description:

Hello!
4.5 million rows are inserted from the temporary table. For each of the
three subscriptions, 712,000 xid-*-lsn-*-*.spill files of 3 GB are
generated. The following lines appear in the work log: ERROR: replication
slot is busy processing with PID. Replication stops working.
What to do?

PostgreSQL 12.2, compiled by Visual C++ build 1914, 64-bit
postgresql.auto.conf
max_connections = '30'
shared_buffers = '512MB'
effective_cache_size = '6GB'
maintenance_work_mem = '512MB'
checkpoint_completion_target = '0.9'
wal_buffers = '16MB'
default_statistics_target = '100'
work_mem = '21845kB'
min_wal_size = '2GB'
max_wal_size = '8GB'
max_worker_processes = '4'
max_parallel_workers_per_gather = '2'
max_parallel_workers = '4'
max_parallel_maintenance_workers = '2'
random_page_cost = '2'

P.S. Replication on PostgreSql 11.1 (with default settings) worked fine with
inserting 4.5 million rows. The xid - * - lsn - * - *. Snap files were
smaller and larger in size. But if there were several inserts in a row of
4.5 million lines, an error appeared: ERROR: out of memory.

Best regards, Dmitry