Eigrp Static Neighbors

For various reasons, we're looking to configure our hub sites with static neighbours instead of letting EIGRP automatically determine it.

Devices are all within the same AS and all local to site. Site has 2 routers and 2 core switches running EIGRP. R1 and R2 both neighbour to each Core Switch but not to each other.

What I've found is if R1 & R2 both learn a route (10.0.0.0/24 e.g.) via a VPN tunnel, the core switches see both routes via R1 & R2(expected). If i then tear down the tunnel on R1, the core switches still see the route via R2 but the other router doesn't. I would expect the Core's to advertise this route to the other router but it doesn't show (Successor, Feasible or neither)

Without neighbouring the routers directly, can you think of what would cause this?