Deleting messages now deletes the full thread

[AC Question]

I recently updated my galaxy s6 edge plus to nougat, and I use the predownloaded messaging app. I usually delete messages every week, I've tried both ways, clicking each individual message, and clicking delete all and unclicking the ones I still needed... mind you i also have locked messages.
Regardless of the route with the update it seems to delete the full thread of messages even my locked ones. I've even tried keeping the thread pinned(I don't actually know what that function does) but I tried that this morning and everything gets deleted.
I don't click the button to delete the protected messages and I don't understand why it keeps doing this.

Please help.

 

[Tim1954]

If you open the messages, long press one message, tap delete. You should get a little circle beside each message. Tick the ones you want to delete.
Pinning message just means that conversation will always be on top of your messages, the first one in the list of you like.