Mib49, looking at what you provided, unless there's a problem with the way Nord's config handles changing between clients (I take it you're using different Nord server addresses?), I can't duplicate what you've described on our 86U, which is running the newest stable Merlin FW release.
Sometimes the 86 can be a finicky beast. If all of your devices are assigned a static IP and all are routed the same way through each of your OpenVPN clients, ie, each Nord config/tunnel, if all you're doing is turning one config off, then turning another tunnel on, ir nothing else is misconfigured or is lingering outside of the first tunnel, you shouldn't have to reboot just to turn off the first client to be able to use all of our devices through the 2nd/next OpenVPN tunnel.
We usually have two concurrent OpenVPN tunnels running, and they come on when the router is cycled on each day. Sometimes we only need one tunnel. At those times, I've seen no problems with our 86 releasing one tunnel and picking up another, again as long as all devices are configured and routed correctly in both configs/tunnels.
There have been cases where using the same UDP port on two tunnels can result in a conflict, depending on your config and how the provider's subnets are laid out, and their/your DNS requests are handled. If you're allowing Nord to handle all of your DNS requests, there should'nt be a reason you'd have to reboot to clear your router, before moving to the 2nd client; the 86 is quite responsive if configured correctly, and the VPN provider handles your DNS instead of using your ISP's DNS or a third parties DNS services, such as cloudflare or Quad9. None of our traffic ever drops to WAN/or the ISP and we're pleased with the our VPN provider's DNS solution. Nord might have some some data caching going on with their end which may be why the first config/tunnel isn't releasing the devices to the 2nd tunnel. That's something we have run into and worked through with our VPN provider.
As long as rebooting clears the cachine or log jam, and your devices work well in either client, it's only a couple of minutes; grab a coffee, take a stretch. Good luck, hope this helps.