Mystery man avatar on Facebook signups/logins

Hi, and thanks for this plugin. On one of my sites I have set it to use a custom avatar rather than the mystery man default, but your plugin always uses mystery man for Facebook signups/logins without their own avatar – looking at the plugin files I see that mystery man is actually included and referenced when the page is output (file: heateor-social-login/public/class-heateor-social-login-public.php, lines 3441 and 3442).

I have tried overwriting that file by editing the src in those lines and including it in my custom theme folder but it didn’t work, so I assume it is not possible to overwrite it. Is there some code I can put in my functions.php or some other way to deal with this? I suspect that this is either another one of those quirks of Facebook that requires you to have written the code in this way, or it’s something that has been overlooked, as it doesn’t happen with any other social login provider and is the only place you use this particular code.

Appreciate any assistance that you or other users can provide.