Just tested this again and it worked as expected.
This time I took more time to pay attention to the steps I did. One of the first steps is to decide which reply button to click and I did not consider that as significant the first time so did not record which one I did. This time I did and used the reply to the entire topic, or as some may think of it as replying to the first post.
It worked this time.
Will try again a few more times to make sure this is reproducible and try and figure out why I did not see it work the first time I tried it.
In another test noticed that the icon will change based on the selected action.
So need to see 
in the upper left after a choice is made.
Used the two different reply buttons and both worked
So for now will take it that the test that failed for me was because the option was not set, which can be checked by looking for the icon.
Don’t want to consider this closed just yet but indicationars are good.
FYI
In the OpenAI site another user at TL3 and me at TL4 have been discussing the option Toggle topic bump; seems that as TL4 I have the option and they at TL3 do not have the option.
As a fan of
the entry Reset Topic Bump Date seems to confirm this.