Actually this is another problem. Ideally, the Guest Network should only be open to the internet via the WAN port. However, this was discussed in another thread a year or so ago. Apparently, VPN connections are open to the Guests.Are you expecting your guest internet traffic to be routed through the VPN or to bypass the VPN and go directly out through the WAN?
If the former, your VPN server could be dropping traffic destined to the different subnet used by Guest #1. Perhaps it needs an additional route/iroute defined.
If the latter, do you have "Force internet traffic through tunnel" enabled or disabled on your VPN client settings? I have it set to "No" so that only traffic going to other VPN subnets gets routed through the VPN. With this configuration everything works fine even for guest #1. If you want your non-guest LAN traffic to use the VPN but the guest LAN to bypass it, then it seems like it would take some custom routing rules to make it work.
My VPN Clients are configured Policy Rules (strict), so only traffic to my remote office subnets go thru the VPN connections. All other, non-local traffic goes thru the WAN port.