Currently if you have Approval process "A" and it is actively being used in a issue approval and you edit the Global Approval Process "A" and make a change to any of the stages it resets the approvals for that stage. I.e. Person 1 approved but after the reset Person 1's approval is not showing. Here is the WF support article that explains it better than I. https://experience.workfront.com/s/article/Creating-Approval-Processes-1001577410#modifying-an-exist...
The purposed idea is to NOT have changes applied to active or previous uses of that particular approval processes. Similar to how if you make a change to a project template it doesn't affect all of the projects that use that project template.