The NI-9862 XNET module requires a special set of drivers to be loaded to the FPGA in order to be able to access the module as an XNET interface, as well as see the interface in MAX. These drivers are automatically built into an FPGA VI when it is compiled if the module is included in your LabVIEW project.
Because the drivers are built in to the compiled FPGA bitfile, they are only loaded when the FPGA bitfile is deployed and ran once on the FPGA (interactively). The FPGA VI itself does not have to remain "running" for the drivers to remain loaded and usable. Compiling a blank FPGA VI that does nothing will work fine to load the drivers.
Another possible cause of this issue is if you use a "Close FPGA VI Reference" or "FPGA Reset" method in your Real-Time code. This will cause the drivers to unload and the interface will disappear from MAX and no longer be usable with XNET.