No actually its not a dynamic page issue, it is apparently a bug with all input fields. So unless the user takes their cursor over the input box and clicks on it, the input does not consider it as the value you’ve set by code.
I was facing the same issue but I managed to solve it using a simple workaround.
Try calling the $w('#input').focus(); after assigning a value to the input or before reading the value of that input. This should solve the issue.