Update for the public record…
1.
I tried both the Better User Search plugin and the code I linked to on a single-install WordPress and it worked fine, but it won’t work on my Multisite, even with the theme switched to default and pretty much everything disabled.
2.
I ran “grep -lr “pre_get_users” *” in both the plugins and my theme folder.
pre_get_users turned up in my Admin Columns Pro plugin – but that plugin is actually disabled.
