As explained by @iagirre with pinpoint accuracy [1]:
"If the officer_assignments are created at 23:59:59 and the rest of the
test is executed after 00:00:00, the dates for the objects and the
`Date.current` (used to check if there are any shifts today) won't be
the same, because the shift will be for, lets say, 07/03/2018 and
`Date.current` will be 08/03/2018, so, there are no shifts."
Freezing time avoids this issue.
Related to issues #2520 and #2521.
[1] https://github.com/AyuntamientoMadrid/consul/pull/1342
4a559ed3cd