From an internal bug report:
I can reproduce it on kevin 66.0.3358.0/10444.0

---
Chrome OS Version  :  R66 10450 / R66 10452.7

What steps will reproduce the problem?
1. Add Koreand language and Korean input method
2. Change to Korean input method
3. Ensure that input mode is latin alphabet. 
   (Korean input method has two input modes and you can alternate it by pressing right alt key. )
   If current input mode is Korean, when you type 'gks', you can see '한'.
   If not, you can see 'gks'.
     
4. Change to tablet mode
5. Check language of virtual keyboard.
   It will be korean keyboard.
   Press "ㅎ", "ㅏ, "ㄴ"

6. Check the input result.   
     

What is the expected output?
The input text should be combined text "한"

What do you see instead?
doesn't combine 'ㅎㅏㄴ'

How frequently does this problem reproduce? (Always, sometimes, hard to
reproduce?)
Always

Typically, right alt key is working as korean/english changing button.
 1. There are no key like "right alt" in virtual keyboard.
 2. Therefore sometimes mismatching is occurred between virtual keyboard and physical keyboard.
    physical keyboard : Korean input method, but current input type is english by press right alt.
    virtual keyboard : Korean input methos, and input type is korean