This was quite a nice one to troubleshoot, turns out there is a new configuration point for per app VPN and iOS devices, at least it was for me.
If you follow the configuration at https://www.citrix.com/blogs/2016/04/19/per-app-vpn-with-xenmobile-and-citrix-vpn/#:~:text=With%20the%20iOS%20per%20app,applications%20installed%20on%20the%20device. you’ll end up with a config that won’t open up a VPN when accessing the browser. Solution for this is to change the default provider type in the policy from App Proxy to Packet tunnel also mentioned here https://docs.citrix.com/en-us/xenmobile/server/policies/vpn-policy.html and explained it means the following:
Provider Type: A provider type indicates whether the provider is a VPN service or proxy service. For VPN service, choose Packet tunnel. For proxy service, choose App proxy.
Hope it helps!