From a real quick search:
In Luke 17:20b-21 Jesus said, “The coming of the kingdom of God is not something that can be observed, nor will people say, ‘Here it is,’ or ‘There it is,’ because the kingdom of God is in your midst (or, within you.)” The Kingdom is of Heaven is here with us now.
And Jesus did appear to many (500???) in the many days he spent on earth after His resurrection. I believe that many count His resurrection as the BEGINNING of the Kingdom, a partial glimpse of the glory of heaven. It makes sense when I think of Jesus as my King. (Of course Satan still has a lot of power).
Neither of that explains the Matthew 16: 27, 28 passages.
They may have seen him, but they didn’t see him with God and all the angels. Nor did they see him with God and all the angels judging every man according to his deeds. Also “Before you taste death”, I’m pretty sure that implies some who he was speaking to would see those things happen before they died. I’m pretty certain that the Bible has never claimed that happened. But it claims that it WILL happen.
The passage is pretty specific and can’t be written off as referring to the spirit of the Kingdom. Even if it did it still doesn’t explain that He said they would see it happen.
1.) Some of the people he was talking to were taken to Heaven or elsewhere before they died.
or
2.) The passage is wrong.
or
3.) It’s misquoted or mistranslated.
I was just thinking that Jesus could have shown some of them the future.
I guess that would count.