Our server level backup software, Veeam, was apparently configured to truncate the Transaction Log. I had no knowledge that Veeam even could interact at the database level until the wise Tibor Karaszi pointed me in the right direction:

(This was configured long before my existence. Side note, I see absolutely no reason why Veeam would even offer this terrible option.)

Changing this to backup the logs themselves (instead of having a maintenance plan do it) would fix the issue. But I prefer letting SQL Server natively manage the backups, so we just disabled the management of this in the Veeam Backup & Replication software.

For reference, this is Veeam's documentation on configuring it to manage the Transaction Log.

Our server level backup software, Veeam, was apparently configured to truncate the Transaction Log. I had no knowledge that Veeam even could interact at the database level until the wise Tibor Karaszi pointed me in the right direction:

(This was configured long before my existence. Side note, I see absolutely no reason why Veeam would even offer this terrible option.)

Changing this to backup the logs themselves (instead of having a maintenance plan do it) would fix the issue. But I prefer letting SQL Server natively manage the backups, so we just disabled the management of this in the Veeam Backup & Replication software.

In the Veeam Backup & Replication software, under the Edit Backup Job window, in the Guest Processing screen, we unchecked "Enable application-aware processing" to disable it.

For reference, this is Veeam's documentation on configuring it to manage the Transaction Log.