I understand your frustration.
Since MATE changed to ayatana indicators you probably discovered that the date/time indicator changed too. It is no longer a gnome(mate) panel gadget but a seperate ayatana indicator so the key changed too ofcourse.
The default setting for 12h/24h for ayatana-datetime-indicator is automatically selected based on your regional settings.
You can still override this by invoking
gsettings set org.ayatana.indicator.datetime time-format '24-hour'
Try to set this and see if it works.
If it doesn't, create your custom format by setting:
gsettings set org.ayatana.indicator.datetime time-format 'custom'
gsettings set org.ayatana.indicator.datetime custom-time-format '%H:%M'
You can lookup the formatcodes you need (like %H %M %s etc.) by invoking:
man strftime
The alternative is to revert to the old date-time indicator instead, by disabling the ayatana version and right-click the panel and add 'clock'.
This would not be the preferred way but if all else fails it might just save your day.