Prioritize bluetooth tether over Wifi on an iPad
I am guessing that the problem might be related to the DHCP server on the WiFi network, telling your iPad to use a 'default router' on its network.

If the DHCP server sends a value for the 'default router', then the iPad (or any computer, for that matter) will use that router address for all traffic that is not within any of the subnets to which it is attached. This is expected behaviour.

If you have access to the DHCP server's settings, try telling it not to give clients a 'default router', and the problem should go away.

Another suggestion is to turn off DHCP in the WiFi settings on the iPad, and allocate yourself a static address. Again, it helps if you are involved with network administration here, as otherwise you might allocate yourself an address that is already in use, and badness will ensue. But you could allocate yourself an address, a subnet mask and DNS server(s) - and do NOT set a 'router' address.