A dynamic rule becomes a static rule triggered by the first decision
As soon as the first decider approves/declines, a dynamic rule will be converted into a static rule to keep the deciders stable during the started decision process to avoid misuse (analogy: you cannot modify the candidates during an election). Also, imagine using a dynamic rule referencing the deciders by a related Jira group and you assign a new team member to this group after an issue has been finally signed-off. Based on the existing approvals, money has been spent, and actions have been taken. Because of the automatic conversion from a dynamic rule into a static rule, the deciders keep the same, and everything is fine. However, if you would still use the dynamic rule, this new member would become part of the group, and her/his decision would be necessary: instead of “finally signed-off”, that issues' rule would become “pending” again until the new group member votes: for all related issues, even backward in time!
Generally, you should stop a decision and restart with a new set of deciders. You can configure this, e.g., using a newly created workflow transition named “restart decision” from the current status into the same status, having a post function configured to set the dynamic rule explicitly again (overwriting any static rule), or using “set to default” assuming you had configured a default value.
Also, as an admin, you can manually modify any static rule by adding another user and his/her role as a comment. Do not forget to add this user to your rule's logic! From my point of view, this approach should be an exceptional case, for example, as part of a business continuity process, and not a standard approach due to too much manual admin work.