RPZ stress test fails to process enough TCP queries on FreeBSD

RPZ stress test on FreeBSD sometimes fails to process enough TCP queries.

Job #3286544 failed on main:

2023-03-31:03:11:38 INFO: Server 'ns4' (rpz) received 8,670,491 TCP queries
2023-03-31:03:11:38 INFO: About 10,800,000 TCP queries were expected
2023-03-31:03:11:38 INFO: Minimum number of TCP queries required to pass is 9,720,000

Only 8,683,231 TCP queries were sent by the two TCP Flamethrowers (4,460,466 + 4,222,765) in one hour.

The average number of queries sent keeps falling from 1500 qps (the maximum set with the -Q option) to 700-740 qps after an hour (TCP1 & TCP2); TCP3 & TCP4 was the more usual case when the test passed.

I don't know if it's BIND, Flamethrower, or FreeBSD.