The displayed list of users liking something is cached in a separate database column for performance reasons. Querying this data live would result in additional queries, in fact one query per liked object including the requirement to sort the results which can easily get slow the more data is present.
There are two possible scenarios in which this list gets updated:
The user revokes the like or implicitly does so by disliking content
Rebuilding data via the ACP offers an option to rebuild this list
Unfortunately rebuilding these lists on username change can have serve consequences, while it would work to update a hundred items, this process would be almost impossible with many hundreds or thousands of liked objects. This is due to the fact that we would need to rebuild every object's cache the user has liked because we cannot reliably determine in which object the user was one of the first three users to like, thus we need to rebuild all of them.