Oh, look! That’s my friend (Kansas guy!) and me in that topic! Cool, but anyways let’s stay on topic; I see that it only happens when you press the reply button.
I reviewed the CSS rule and unfortunately this is not currently possible with pure CSS. I believe I looked at this issue when I wrote the rule originally and it is simply a limitation of doing this without any further engineering effort.
Discourse does not provide enough information in the DOM by default for CSS to know when an expanded reply belongs to the OP. To enable this, at minimum, Roblox would need to write code to add additional classes to the DOM. It is my understanding that this is not currently a priority; this OP tag was added as a quick and cheap UX boost.
For the time being, until there are resources available for Roblox to invest engineering effort in expanding the frontend with either sufficient CSS tags, or an in-depth system to provide a more feature-complete solution, this issue will remain.
This is not really a big deal as far as I understand. You’ll have to live with it for now.