Is it okay to add mention of a grant in the acknowledgements at the proof-stage when the grant started after the paper was submitted for publication? Some time ago, I have send a paper for publication at a respected journal. The peer review took some time, and eventually the paper is in the production stage. The paper was submitted when we were working at a grant proposal. Some months ago,  the grant proposal was accepted and currently we receive funding from the grant. Since the paper was submitted before the grant was active, the question would be if it is fine if we add acknowledgements at the proof stage for the current grant.  I add that for dealing with the referees remarks, we have also worked at the paper, work that correspond to a time when the grant was active.
I think you need to look deep within yourself and do some introspection about the reason why you are thinking of adding an acknowledgement.

If the reason is that you genuinely believe the work you did on the corrections to the paper was substantial enough that the grant agency truly deserves to be acknowledged for its support in making the paper come to a successful fruition -- sure, go ahead and add the acknowledgement. No need to overthink the issue.

But if on the other hand the reason is that you are essentially looking for an excuse to add an acknowledgement so that you can later list the paper in your final or periodical report to the funding agency -- perhaps due to a feeling of insecurity about your ability to produce more substantial work later on in the life of the grant for which you can add a more sincere acknowledgment -- in that case, no, sorry to disappoint but I think such an action would be borderline unethical (and in my personal opinion falling on the wrong side of the border, though not by much), and would also look quite ridiculous should anyone (e.g. the program director at the funding agency, or a panel reviewing a future grant application) later scrutinize your report about what you did for the grant.