This is a bit trickier than it might seem at first. Right now it’s only writing when the value of the expression has changed. In this situation, it doesn’t know what the value of the writeback address is, so it can’t check and write any time they’re different. The possible solutions would be 1) Subscribe to that address and write when they’re not equal or 2) Write each time.
The third option would be a transaction group that let you easily map tags to tags. This has been requested a lot, and is hopefully going to make its way into 7.3. Right now, you can work this up one way or another, but it might take a bit of work. For example, if you want your expression to be in a SQLTag, you’ll need to make a group that has a reference to both the source and target tags, and then an expression that references the first and targets the second.