After the upgrade, however, a single press of the next input method hotkey does not immediately enable hangul input; A new menu item called "hangul mode" must also be toggled separately by mouse on the ibus tray icon for hangul to appear.
To fix this this issue, right click on the ibus tray icon and navigate to ibus-hangul preferences -> Input Method -> Korean - Hangul
Now pressing the Hangul toggle key once should enable Korean input without having to click "Hangul Mode" with your mouse after right-clicking the ibus tray icon.
Another issue is that the Alt_R no longer seems to work as the Hangul toggle key; although you can manually specify Alt_R as the next input method toggle key in ibus-setup and ibus-hangul, once you enter hangul mode with Alt_R you cannot switch back to English by pressing the same key (Alt_R) as was true before the upgrade. My temporary workaround is to use Shift+space as the IME toggle key in both ibus and ibus-hangul.
Also there is a typo in the IME toggle key dialog for ibus-hangul 1.5.0:
"Press any key which you want to use as hanja key"
"Press any key which you want to use as hangul key"
The hanja key (for Chinese character input in Korean) is usually Ctrl_R (right side Ctrl) or F9 on Korean keyboards, whereas the hangul key is usually Alt_R.