bridging tinc router mode network and switch mode network

[pjv]

I am running tinc v. 1.1pre (truly as I read somewhere, “one of the internet’s best kept secrets”) on some consumer home routers flashed with tomato firmware. I have a whole network of these, but for the purposes of this question I will focus on just three

Router A (subnet 192.168.5.0/24) is connected via a standard tinc “router” mode network with Router B (subnet 192.168.15.0/24).

Router B, in addition to its connection with Router A in “router” mode, is also connected to Router C via a “switch” mode tinc network. Router C’s IP address is 192.168.15.101. The switch mode network is using a separate device in tap mode and is configured manually on a different port from the “router” mode network.

Routers A & B can mutually ping each other and I also have iptables forwarding rules so that any devices connected to them can ping eachother across the tinc mesh as well.

Routers B & C can mutually ping each other and all broadcast traffic from each side of the bridge passes over (i.e. one can see windows network shares on devices connected to router C from devices connected to router B).

The problem is that Router A and Router C cannot see eachother (nor any of their connected hosts see the other’s connected hosts). I would have expected that since the switch-mode network is functioning at layer 2 that Router C would be visible to Router A (or any hosts connected to Router A) on the router-mode network just like all the hosts that are directly connected to Router B. What am I missing?
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