Rails scope where clause appears to be cached

Using Rails 3.2.22.5, Ruby 2.0.0-p648, I have this query, which among other things is supposed to return patients less than 18 years old:

scope :should_be_underage, joins("left join patient_status on patient_status.patient_id = patient.patient_id left join patient_state on patient_state.patient_id = patient.patient_id LEFT JOIN patient_account ON patient_account.patient_id = patient.patient_id INNER JOIN account_covers_disease ON account_covers_disease.account_id = patient_account.account_id").where("patient.dob > ?", 18.years.ago).where("((patient_state.patient_particip_status_id <> 7 and patient_account.patient_account_status = 1 and account_covers_disease.disease_id = 2) OR ((patient_status.patient_status_id not in (9, 36)) and patient_account.patient_account_status = 1 and account_covers_disease.disease_id = 1))").uniq

This runs fine, but a few days ago it seemed like the results of where("patient.dob > ?", 18.years.ago) had been cached. We were getting patients returned who were four days past their 18th birthday. When I artificially set one of those patient's birthdays back a day, they fell out of the results. When I returned the patient to the original birthday, they joined the results again. So it seems that the query as a whole was not being cached.

Five days later, I tried again, and now the query was returning patients nine days past their 18th birthday, i.e., the same group of patients. I tried the artificial birthday reset test again and again it worked. It's as if just the value of 18.years.ago had been cached, but the query otherwise runs dynamically.

When I rebuilt the codebase, the problem went away, but that's obviously not a production solution. Does anyone know what's going on here and how I can correct it? Thanks.