Let me introduce you to Gildas.
Like Bede, Gildas was a 6th Century monk who was born in the year of the Battle of Mons Badonicus, or, The Battle of Mount Badon, as we now know it. Mount Badon is, of course, the famous battle where King Arthur halted the Saxon invasion for a time.
However, Gildas give the word 'vague' a whole new meaning. He gives us very few names and no firm dates. He misses out chunks of history if they do not serve his purpose or his message.
So what does he say about Arthur?
The only 5th century person he does talk about is a man called Ambrosius Aurelianus.
Gildes is very vocal about many things, but not Arthur. He doesn't mention him. At all.