To achieve read new, the LogFormatter is used to display events specific just to CheckUser (such as logins). While I cannot seem to re-produce this locally, on WMF wikis store failed login attempts when the username is an IP address.
Example row from enwiki which currently uses the old schema and thus actiontext in the wiki's content language (IP address removed for privacy reasons):
The LogFormatter expects that the username meets the RIGOR_VALID constraint, which IP addresses do not. As such, a InvalidArgumentException would be thrown and the results list would not display.
As such, to ensure the rows can be displayed usernames that are IP addresses should be accepted by the log formatter and displayed as IP addresses.