I will add much needed photos to this rather very rare bug we are experiencing on our phones. As what the author already mentioned, the main camera was working prior to .78 update, that is, all 3 camera sensors were usable. After the said update including the latest .98, the stock camera app as well as apps that use the cameras such as Facebook, WhatsApp, Viber, and other third party camera apps only "see" the wide-angle and macro sensors and thus set them as default cameras. As shown in the above photos, the slider for wide-angle/main sensor is stuck and set only to wide-angle. Under Pro mode, it can be seen that the focus mode is greyed out because the camera app is forced to use ONLY the wide-angle sensor.
Info apps that can be downloaded from the Play Store can still "see" the main 64MP sensor since they still report its presence, even showing the hardware specs of the said sensor, thus, THIS IS NOT AND SHOULD NOT BE A HARDWARE ISSUE. Asus claiming it is not a software issue is rather hard to believe.