Javascript in HTML Area - inconsistent behaviour

I am looking to update some functionality that uses custom Javascript in an html area and am seeing unexpected results in both Classic and Fluid (we are on PT8.58.07) as to when the HTML area is refreshed and therefore when my custom Javascript is run. This can be be recreated as follows:

Test 1

• Add an HTML area to a classic page that contains a field that does not have deferred processing enabled
• Set the value to Constant and enter the following:

<script>
  alert('Hello World');
</script>

• Leave Respond Only Once unticked

Result: My alert displays when I first enter the page and any time there is a server round trip (eg on field change or button click). This is as expected