Can't silence notification sounds with power key after update

[Lex76]

Hi please can anyone tell me if they are experiencing the same problem?
I've just done a system update on my Xperia Z5 Premium. Before updating I was able to silence all my notification ringtones by pressing the power button but now every email, text, WhatsApp message, etc will only be silenced by undoing my phone case and reading the message or dragging down the notification from the top of the screen and swiping it away to read later. It's really annoying as I've never had this problem before on any previous update! This is my first post on this forum and I'm a complete technophobe so please give any technical help as if you were talking to a 5 year old lol!
Cheers,
Lex.

[Uliwooly]

@Lex76

what if you press the volume down ?

[Lex76]

Hi,
Many thanks for your reply but no I did try pressing the volume key up and down as you suggested but it still won't silence it. I'm still having to use the fingerprint or PIN number to unlock it before I can drag down the message from the top of the screen and swipe it away or open it before the message tone stops. I've had this for over two weeks now and I'm still not getting used to it, it's annoying! Is it just my phone or is this a known bug?
Thanks,
Lex.