Ajax link is not working when Content is Bookmarked or when removed.

Hi all,

I have recently upgraded one of our web application to Drupal 8.5.3 with the Flag contrib module updated from v8.x-4.0-alpha2 to v8.x-4.0-alpha3.

I had bookmark flag created for new content types. The bookmarks worked well in v8.x-4.0-alpha2 where I have AJAX link type for the bookmark flag.

After the latest version 8.x-4.0-alpha3, on all the content page when I click on 'bookmark', the text still remains as 'bookmark', does not change to 'Remove bookmark' (which previously worked well with ajax). However, it does the actual bookmark that is when I reload the page, it shows the 'Remove bookmark', but no longer works with ajax as it used to.

Same happens on the 'My bookmarks' page when I click on Remove, it is not shown visually through ajax. However, on page reload the bookmark is no longer listed.

I got the below error in the console, when I click 'bookmark' or 'Remove'.

Has anyone come across this issue? Any pointers will be much appreciated.

Thanks!