Comment 3 for bug 1966181

A-ha! I wasn't seeing things after all. Our test images install the "systemd-timesyncd" package (as we also run tests against that), and that removes the chrony package and installs the mask:

# apt install systemd-timesyncd
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
The following packages will be REMOVED:
  chrony
The following NEW packages will be installed:
  systemd-timesyncd
0 upgraded, 1 newly installed, 1 to remove and 0 not upgraded.
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dpkg: chrony: dependency problems, but removing anyway as you requested:
 systemd depends on systemd-timesyncd | time-daemon; however:
  Package systemd-timesyncd is not installed.
  Package time-daemon is not installed.
  Package systemd-timesyncd which provides time-daemon is not installed.
  Package chrony which provides time-daemon is to be removed.

# ls -l /etc/systemd/system/chrony.service
lrwxrwxrwx 1 root root 9 Mar 24 12:16 /etc/systemd/system/chrony.service -> /dev/null

Mystery solved!

So, sorry for the noise!