Korean Keyboard not working properly

When I type 가감법, sometimes it is typed like 가가감법, which first typed character typed again. And sometimes, although there are only 3 characters in the textbox, another character suddenly appears after pressing backspace 3 times. For example, I pressed backspace while having a focus on the textbox with text “가감법”, the textbox doesn’t become empty and something other character like “를” or “한“ appears while still pressing backspace.

It doesn’t seem to be a problem caused’ by network lag. 

I’m using iPad Pro 3rd gen 11