I imagine that your Computation Rule is functioning correctly in the Ad-hoc test and Form Model Preview. This is because clearing Field Values with a Computation Rule is fully supported in the Front-end.
Please have a look at this thread for more information about clearing Field Values in the Backend, How do I recompute an existing document in the backend? (solution).
We also created a Requirement Ticket, A12-16238, which has been completed for 2024.06-ext2. This means that when your project migrates, you will be able to clear fields when a Document is updated in the backend and no precondition applies.
Please note, you will need to set up Data Services correctly using:
mgmtp.a12.dataservices.documents.computation.enabledCleanupErrorAndNotComputedValue=true