To acknowledged a message (note as being read without replying), you click on the "You have not yet responded" at the bottom of the message that was a reply to your message. That toggles the "You have not yet responded" to "You have acknowledged this reply".
There is no "acknowledge" button.
As best I can see, the acknowledgement process is still working correctly.
Please be familiar with the various topics and other links in the "Essential Links", found in the top of the page menu. Amongst other things, this is where to find where to report various forum problems.