This is my first time using an iOS phone, which is an iPhone 16 Pro, and it has already been used for 6 months. I’ve noticed that the keyboard sound is sometimes really loud, sometimes normal, and sometimes very soft. Is this a bug or an issue with my phone?

It's a bug in iOS. There is only one volume control setting for the entire phone, and some system sounds may switch between the global volume setting (which I tend to make almost max, to be able to hear the ringtone in my pocket) and the default (softer) setting used for typing on the keyboard.

This a bug that has been around forever.

Same with notifications sound as well.

My emoji keyboard sometimes bugged too, the emoji won’t appear especially when I’m on TikTok.

Have the issue resolved ? probably due to software bug I would recommend restore the phone.