The way youāve described it doesnāt sound like itās a current feature of the review queue.
You generally see the view full conversation option on flags that also create a PM to the moderators group (the āsomething elseā and āillegalā flag types). Pressing that will take you to the PM itself rather than the reviewable in the queue.
Thereās no time delay on when that option becomes visible, it should be there immediately on the flag landing in the queue. And the spam, off-topic, and inappropriate flags donāt have that button at all (as they donāt generate a PM with extra detail in).
Does that match up with what youāre seeing in your Review Queue?
For the default ones of spam, off-topic, and inappropriate, I donāt believe itās possible to edit their behaviour.
Though there is the option to create custom flags where you can set whether they require extra detail (and therefore would create a PM as well as a reviewable in the queue, which would give them the view full conversation button). I think it should then be possible to disable the default ones and replace them with similar custom versions that have the āPrompt users to provide additional reasonsā option checked.
You can find the custom flag options under the moderation section of your admin area (/admin/config/flags)
Do you want to reply to the user who flagged the post or do you want a conversation with staff members?
I think usually you use the staff category for the latter. As a workaround you could flag the post as āsomething elseā yourself.
Maybe Review with reason would also help you but I think it only works for comments while you take action
The issue first came up when one of the mods pointed out that they wanted to comment on a flag without taking action but that the view full conversation option was not there.
I will look into the custom flags option mentioned by @JammyDodger
Is it possible to do that after something was already flagged?