Why are some Corporate Card transactions automatically becoming inactivated?

Every month, some number of Corporate Card transactions are automatically designated with a Validation Status of "Inactivated transaction", and we can't figure out why. (Yes, we are able to successfully "activate" them by running the "Manage Historical Corporate Card Transactions", thankfully.)

• The employees and their cards were and are still active.
• The transactions are all similar to other transactions which were successfully validated.
• The upload generally also included similar transactions for these same employees that were successfully validated.
• Happens with both Employee cards and group cards.

Documentation, Cloud Customer Connect, and the Knowledge Base provides no information we can find about what might cause this to occur automatically.