Hi Inumaki,
if it’s a hole, you can go into Edit Mdoe, select it (ALT+Cliick on a border Edge of that triangular hole) and F to ‘fill’ it.
When it occurs during Beveling (and changes size, when chaging the Bevel), it’s probably not a hole, but overlapping Geometry. Reduce the Bevel amount untill it’s gone.
