What does that mean?
Hi and welcome to the forum 
Why am I seeing this message?
In the lastest versions of Threema (unfortunately you have not specified which version you are using) this option is enabled by default.
Regards Ingo
I'm just a bit confused why I see it only in one chat and not the others. Do you happen to know why?
Hi
not sure. But maybe PFS is explicitly disabled for those, or the other partners are using older versions of Threema.
Regards Ingo
Understood, thanks
Additionally, if no interaction (message exchanges, profile picture updates, ...) is done with the others (regardless if in a 1:1 chat or in a group), there will be no "status update" sent between you and your contact and therefore you won't see this information in the chat.
So this means, once your contact also updated to 5.1 (for Android) or 5.4 (iOS) or later and you send/receive a message to/from this contact you also will get this popup.
