Any help here?

This is an old problem on WP. Members with just numbers or numbers at the beginning of their user names cause the system to parse to the member_id number recorded in table rather than the member_name. *I have not found an automated way to solve the problem.

When I get a new registration I edit their nickname to add an underscore at the beginning which solves the issue. Like so _98765.

You can then run a snippet to block users from changing their Nicknames so that they don't change it back and break the fix. You will have to switch it off every time you get a numbers registration you need to edit.

add_action( 'wp_head', function () { ?>
	<style>

		#wpf-profile-account-form > div > div.wpf-tr.row-0 > div.wpf-td.wpfw-1.row_0-col_0.wpf-row-user_login.wpf-row-display_name.wpf-row-user_nicename.wpf-row-user_email.wpf-row-title.wpf-row-groupid.wpf-row-location.wpf-row-avatar.wpf-row-about.wpf-row-occupation.wpf-row-signature > div.wpf-field.wpf-field-type-text.wpf-field-name-user_nicename.wpf-field-required > div.wpf-field-wrap {
      pointer-events: none;}

	</style>

* There is a plugin that can restrict usernames but it seems overly restrictive https://wordpress.org/plugins/restrict-usernames-emails-characters/ I have not tested it though.

I'm having this same issue, and can confirm that affected members do have numbered usernames because they are autogenerated from a third-party system. 

Note that when @verek explains about the nickname, he means the one in the wpForo profile, not the nickname in the backend user profile.

I checked a couple affected users and both had alpha nicknames in the backend user profile but when I checked the frontend forum profile they were numbers. The private messages only started working properly when I changed their wpforo nickname.

So the question that begs to be asked, how can the nicknames be fixed on a bulk level to something like "@firstnamelastname" after the fact? Or at the very least not having the system create numeric nicknames in the future.