When user running the action has created or modified a page, getPage or getContent will not return the correct creator or modifier fields. If the content is created/modified by some other user (not the user running the action), the correct name appears to be returned.
The incorrect name displayed appears to be a generated name: User xxxxx where the last 5 characters of the Atlassian id are used.
This is an Atlassian bug that appears to have been released after 2019-08-04.
This has been reported to Atlassian support.