Doh! So sorry @sengsberg That is my bad… You don’t write to the same topic namespace because if you did, the tag would change and publish the new value on the same topic which you subscribe to and publish the change … and so you would have an echo race through the broker.
You need to put write in after the base topic.
I just did this example in Node-RED because its easy to see/show. Of course its your web app that has the publish topic in it.