Could you show that “discordapp.com” (the old domain) is allowed? Otherwise, this wouldn’t be a bug as you are expected to use the shortened invite links.
But does ROBLOX allow “discordapp.com”? It was their old main domain, so if they didn’t allow that, then it explains why they don’t allow “discord.com” now. If they don’t allow it, then it’s not a bug because you aren’t intended to use it anyway.
No. The old domain would’ve been “discordapp.com/invite”. If that isn’t allowed, then this isn’t a bug because “discord.com” isn’t supposed to be added.