I’ve just hit this issue in Hibernate 5.4.12 Final (upgraded from Hibernate 4.3.x) against Postgres, same symptom - Hibernate attempts to insert a PK that is the DBs’s nextval - allocationSize (which defaults to 50 on the JPA @SequenceGenerator annotation). We didn’t have the same issue with Hibernate 4.3

insert into BACKFILL (HTML, IS_USED, ID) values (NULL, 'FALSE', 12029) was aborted: ERROR: duplicate key value violates unique constraint

...

psql (11.5 (Debian 11.5-3.pgdg90+1))
Type "help" for help.

test=> select nextval('backfill_seq');
 nextval
---------
   12079
(1 row)

Setting hibernate.id.optimizer.pooled.prefer_lo to true as mentioned by the original reporter also fixed the issue for me. I will try to attach a unit test to reproduce when I get time.