using kernel 4.16.0-0.bpo.2-686-pae, the hostapd update to
2:2.7+git20190128+0c1e29f-4~bpo9+2  causes wlan0 to no longer work

reverting back to 2:2.4-1+deb9u4 , allowed hostapd to run again correctly

/etc/network/interfaces
"auto wlan0
iface wlan0 inet static
  hostapd /etc/hostapd/hostapd.conf
  address 10.9.9.1
  netmask 255.255.255.0
  broadcast 10.9.9.255
"

fwiw, the module the wireless chipset is using is ath5k

observation:
- ifconfig lists wlan0
- A client scanning the AP will see it appear for a couple of seconds
before it disappears, even though ifconfig still says that wlan0 is
still up.

the issue is replicated for each of these release, tested multiple times
and the result is the same for hostapd never working correctly with
2:2.7+git20190128+0c1e29f-4~bpo9+2.

Hi,

I’m sorry for hostapd not working for you, but I’m afraid I cannot
provide support for backports to oldstable. Unless someone figures it
out and provides a solution, I cannot fix this for you.

a user can revert hostapd just using apt-get install
hostapd=2:2.4-1+deb9u4
"apt-cache policy hostapd
hostapd:
   Installed: 2:2.4-1+deb9u4
   Candidate: 2:2.7+git20190128+0c1e29f-4~bpo9+2
   Version table:
      2:2.7+git20190128+0c1e29f-4~bpo9+2 500
         500 http://debian.mirror.iweb.ca/debian stretch-backports/main
i386 Packages
  *** 2:2.4-1+deb9u4 500
         500 http://debian.mirror.iweb.ca/debian stretch/main i386 Packages
         500 http://security.debian.org stretch/updates/main i386 Packages
         100 /var/lib/dpkg/status
"

^ the 2:2.7+git20190128+0c1e29f-4~bpo9+2 release was coming from
stretch-backports

https://wiki.debian.org/LTS
"Debian 9 Stretch i386, amd64, armel, armhf and arm64 (to review before
start)   2020 to June 2022"