The above issue persists, though this is not a major concern as all other functions of the plugin are operating well.
I have logged in/out on my own site 6 times and the other sites I administer twice each, yet there is no change.
Also, the "Sucuri WordPress Integrity Checks" section "Administrator Users" lists Username and Email, but no IP Address or Date & Time. There is only one entry, mine, even though on my site I have logged in/out 6 times.
The blue "Show All Results" button on the "Sucuri Plugin Last-Logins" window still disappears when clicked.
I disabled all the other plugins on my site, cleared cache, and logged out/in twice, but no change.