Knot 2.5.3 not flushing zones with "knotc zone-flush"

While testing different configurations for bug #526 (closed) I set zonefile-sync: -1 to force knot to keep all zone data in the journal, and never write out zone files. The documentation suggests that I should still be able to flush a zone manually if I want to. So I ran knotc zone-flush ipv6actnow.org. In the log I see:

2017-07-22T08:50:00 info: [ipv6actnow.org.] control, received command 'zone-flush'

However, there is no zone file in the storage directory (/var/knot)

Then, I tried knotc zone-flush ipv6actnow.org +outdir /tmp. This time, as expected, knot wrote the zonefile into /tmp.

So finally, I tried knotc zone-flush ipv6actnow.org +outdir /var/knot and explicitly gave it the path to the storage directory. This time, the zone file was written.

So it looks like there is a subtle bug that prevents flushing the zone to the storage directory when zonefile-sync is set to -1.