GrapheneOS doesn't change anything with Orbot or VPNs, they aren't bundled in the OS. Most likely Amethyst's Tor implementation is leaky and it needs to prevent falling back to non-Tor if the setting is active. A split tunnelling VPN mode also won't make a difference if it isn't taking your OS VPN slot. The app needs to make an effort to reject it.
Go to Amethyst and generate a new npub, turn off Tor in Orbot and enable Tor in Amethyst. It will still let you post even when it should refuse to connect. This is on the latest build of GrapheneOS. I also have recordings of me turning on Tor when I don't have Orbot installed and it allowing users to post.
nostr:npub1gcxzte5zlkncx26j68ez60fzkvtkm9e0vrwdcvsjakxf9mu9qewqlfnj5z anyway someone can help? Would be open to trying to figure it out.