[exim] Exim not obeying "delay = " in acl_smtp_connect

Apologies for posting for the third time in three days. I have the
following acl in acl_smtp_connect, which appears to be ignoring
completely the "delay =" setting:

drop message     = Reverse DNS record incorrect or missing
        ! condition = ${if eq{$received_port}{587}}
        ! verify    = reverse_host_lookup
        delay       = 600s

The acl is working, I can see the messages in the logs, but the
connections get dropped almost instantly - maybe 1-2 seconds max. I've
tried DENY instead of DROP, I tried 3m instead of 600s. I've even tried
moving it to acl_smtp_helo, and it does the same thing. I've googled,
but all the results refer to the standard use of DELAY =, nobody seems
to complain that theirs is not working. I am on Exim 4.89

Edit: in fact, I just realised that none of the acl's in acl_smtp_helo
obey the delay either.