Hmm if you rediscover the NDI source, or delete the NDI input and then create a new one, rediscovering your NDI source, does the connection reestablish?
If I recall correctly, the GH5 cameras are by default set to automatically detect the desired resolution from the HDMI output. It is possible that when the Pearl-2 rebooted after the firmware update, the GH5 and Pearl-2 couldn’t negotiate a resolution, Have you tried disconnecting and reconnecting the GH5 from the Pearl-2, or trying a different HDMI input port as a test? Does changing the output resolution from the GH5 help?