it sounds like the page is refreshing, this will certainly cause a loss of connection to the CNC once the gcode that’s qued on the machine istelf is complete then the cnc will stop and the Z axis will begin to slowly lower due to gravity… OR if it’s engaged in a cut, the motors will not hold position and the router spinning will just cause it to slowly drive itself around randomly…
I’d suggest opening another tab or browser window and previewing the simulation in that other one while leaving the one that’s running the carve to keep doing it’s thing, that way it doesn’t refresh on you. That said, personally I always download the gcode, then use a different software to send the gcode to the cnc, one that is not browser based which ensures it doesn’t refresh like that, ever… (OpenBuilds Control is the sender software that I use)