Some months ago, I used Tailscale to remotely connect to a PC with B4A installed (my PC was at home) using my phone. Both my home PC and my phone were on the same Tailscale network.
It worked perfectly. Tips:
The phone must be connected to Wi-Fi
Tailscale must be active
B4ABridge on your phone will show the IP address from the Wi-Fi network, not the Tailscale IP assigned to your phone, but it will work anyway
The Connect menu in B4A will NOT show the Tailscale IP assigned to your phone; you will have to manually type the IP address assigned by Tailscale to your phone using the 'New IP' menu item
I have successfully connected B4ABridge with my home computer when I was using TailScale in the past. B4ABridge reports that it is not connected but it is and it works. Either way B4ABridge is a port listening and it does not matter at all how you will get there. What it matters is to connect to this port.
Have you tried enabling routing in Tailscale so you can point to the IP address of the local PC/device/Wi-Fi?
I remember doing some testing a while ago, and it works, but you need to know the IP address of the receiving device.