LG 34G850-B not accepting commands about ddcutil HOT 1 OPEN

Feature x60 (Input Source) is problematic on recent LG monitors. See the very long discussion on issue #100, where your monitor is specifically mentioned. It may have to do with how Picture by Picture is controlled using manufacturer reserved features. Some users have tried to figure out the logic by experimentation. See for example, issue #163.

Feature x60 is also peculiar in that on some monitors reading the value succeeds no matter what the input source, on others it succeeds only if sending the command to the current input source.

As background, setting a feature value in DDC entails sending a request packet. If DDC reports success, it only means that the request was received by the monitor, not that the monitor successfully executed the request. There is no reply packet. For this reason, by default, setvcp explicitly reads the value after setting it. (This behaviour can be turned off using the --noverify option.) However, for feature x60 automatic verification is not performed, because, as noted, it is commonly the case that it is not possible to read values once the input source has changed.

Note also that the capabilities string returned by a monitor is often inaccurate. ddcutil reports the string for informational purposes using the capabilities command, but does not rely on it. For example, your capabilities string reports feature x15, which is undefined in the Monitor Control Command Set spec, and is not in the manufacturer reserved range xF0-xFF.

from ddcutil.