Shared / external endpoint advertised routes unavalable

Tailscale version 1.3.0
Your operating system & version: Debian 10 and Arch Linux

I have an issue accessing advertised routes on a shared device.

I have two tailscale accounts (lets call them ‘Account A’ and ‘Account B’)

Account A has 3 ‘Machines’ M1 M2 and M3
M1 has 2 routes advertised - and
M2 and M3 can access systems on the subnets advertised by M1

Account A has shared M1 with Account B
Account B systems start the client with the --accept-routes option
None of the Machines on Account B can access the systems on the subnets advertised by M1 on Account A though they can access M1

Is this because sharing routes across accounts is not an implemented feature? Or do I have this misconfigured?