Locked hoppers will transport items into hoppers below them.
Steps to reproduce:
Create a hopper with another hopper under it
Lock the top hopper
Place an item in the top hopper
Results:
Items will be transported into the hopper below
Expected Results:
Items will stay in top hopper. If the hopper is facing into a chest and the hopper is locked, it will not transport items.
The hopper below is pulling the item out of the hopper above. Locking a hopper does not prevent other hoppers from pushing into it or pulling from it. See MC-75569.
Resolving as "Cannot Reproduce" since the way you describe the issue is not what is actually happening. The locked hopper is not pushing. Instead, the unlocked hopper is pulling.