Editing a reply message which didn't mention makes it mention

Summary

If you send a reply message that does not mention, then edit it, it will make it mention. This is not normal behaviour.

Details

No idea why that is, I reckon @VelvetThePanda and @ProfDoof will have some input as they are the ones who originally implemented the feature in D#+

Steps to reproduce

[Command]
public async Task TestReplyBork(CommandContext ctx)
{
    var msg = await ctx.RespondAsync("nope"); // sends a non-mentioning reply by default
    await msg.ModifyAsync(x => x.Content = "yep"); // now it mentions
}

Notes

This has to be resolved before 4.0 release.