After doing an upgrade, a lot of reth units or subinterfaces are missing.

• Some units are present but a lot of them are missing.
• No traffic flowing through those units
• Reth interface and its childs are up/up.
• User upgraded from 19.4 -> 21.2.

Upgrading from 19.4 to 21.2 is not a supported jump. It is important to follow the upgrade path to avoid these types of unexpected software misbehaviors.

After doing a rollback, those reth units that were missing were up again and passing traffic.