Find Member feature for addressing PMs or mass emails gives no selection boxes when restricted to members whose names start with a particular letter

If you start a new private message, click Find a Member and then click a single letter of the alphabet to narrow the list down to members with usernames starting with that letter, the resulting list no longer includes checkboxes that allow the author to select the members to address the pm to.

This also happens when an admin tries to use the Find a Member feature to address mass emails, so it's not something related to the UCP or PMs code in particular but is instead related to the memberlist search feature.

This problem is new starting with 3.0.10.

I will try to find the bug this weekend and report a solution if no one has done it by then.