I am experiencing the exact same issue (which is a very frustrating one). I reached out to MSDN support who ultimately forwarded me here. This is actually a bug and should really be fixed as Office js works as expected in Desktop versions of Word but does not work the same in Word Online.
amelehy
Feedback by UserVoice
I am experiencing the exact same issue (which is a very frustrating one). I reached out to MSDN support who ultimately forwarded me here. This is actually a bug and should really be fixed as Office js works as expected in Desktop versions of Word but does not work the same in Word Online.
Here is my MSDN conversation for reference: https://social.msdn.microsoft.com/Forums/windowsapps/en-US/be1de64a-29a0-4225-be71-6c8067cc41a2/word-online-binding-setdataasync-method-not-working-for-html-coerciontype?forum=worddev