valid username getting blocked

Using Wordfence version 6.1.15, with “Immediately lock out invalid usernames” enabled. This seems to work as expected except that one particular user that is getting locked out from logging in with a valid username. The alert email says “…A user with IP address xx.xxx.xxx.xxx has been locked out from the signing in or using the password recovery form for the following reason: Used an invalid username ‘xxxxxxxxxxx’ to try to sign in…” but the username listed in the alert is a valid username.

According to the user, this does not happen every time they attempt to login, but it does happen a majority of the time. Is there any way I can debug this?

Thanks!