Re: Helppppp…facebook timestamp
That helps. Here are some more that are more in tune for iOS:
4 Common Workarounds to Hide Last Active on Facebook ~ TricksStar
Another workaround but it costs money:
https://www.igeeksblog.com/how-to-disable-turn-off-facebook-messenger-seen-on-iphone/
Just so you know there are a lot of posts online around this and specially Facebook and iPhone so you are not alone in this. Hopefully Apple or Facebook will come up with a solution for it.