This is probably a Sonicwall thing to be honest, the Sonicwall has access rules that does the port forwarding from the public interface to the internal interface, the problem is, a device is internal and trying to talk to the public side IP, so it is essentially going in and out of the router and if the Sonicwall is not setup properly it gets really confused with the port forwarding (an internal device talking to it's public IP, which ties to another internal device). This gets even more complicated if multiple VLANs are implemented and the SonicWall manages them all, such as an internal private WiFi VLAN device talking to the public IP that is forwarded to another, different, internal VLAN device...
We have run into this will various applications using SonicWall routers for mobile users that go inside and out of the organization, unfortunately I am not a SonicWall expert, so I can't really tell you what to do... We stopped selling SonicWall devices a few years ago due to issues with VoIP and SIP in a variety of platforms so it's a break/fix kind of thing now for the few remaining customers that have them, any new or upgraded installations are moved to Meraki.