They are just out right stealing BTGs there is no excuse for not crediting people's account.
I'm with you mate, but please bear in mind that BTG (unfortunately) lost its reputation after a stream of security problems.
BitFlyer is taking advantage of that.
The only important thing to do is to bind them to this cristalline endorsement from BTG:
https://bitcoingold.org/vulnerable-wallets/After this we can signal BitFlyer for unfair commercial practice.
Which I've done already.