QUOTE(howszat @ Jul 10 2019, 09:50 PM)
Ok, more info.
That spot is between 2 side tooth. 1st good tooth, and 2nd tooth the dentist has been putting a crown on which Stage 5 (or 6?) before the final crown. He's done all that well because before the tooth was painful. Now the pain is gone, and I can bite on the tooth. But it is still as the last stage before the crown. The dentist is baffled because he said he cleaned out the tooth thoroughly.
Information about the tooth. According to x-rays, the tooth has 2 roots that have a joined channel. So the channel cannot be cleaned out thoroughly. So he filled the root with "chemicals" which will kill any bacteria. As far as he can tell, the tooth is ok.
But the problem spot is not on the root-canal tooth. It is somewhere in between the other tooth. Is the problem the root-canal tooth, or un-related problem?

Here a picture for your reference,
From what you're telling me, it sounds alot like a gum problem. It doesnt not happen inside the tooth but on the outside which affects the supporting structure of the tooth namely the gums and the bone that are holding it.
Your dentist did a root canal that removes the pulp of the tooth which will render the tooth pain free cause it does not have anymore nerve endings. It is normal that there is no more pain on the tooth and allow you to bite.
He has cleaned the inside thoroughly but maybe the outside of the tooth hasnt been cleaned yet.
Its better to get a second opinion somewhere in this case. It hard to diagnose without an x-ray for your case. And your dentist telling you that it cant be cleaned thoroughly is kind absurd.. No matter how the tooth anatomy is, we are supposed to clean it out thoroughly when we do a root canal, and with you having gum issue.. its almost impossible to for him to not notice.. except he chose to ignore it..
unfortunately.. a gum treatment takes a longer time and need constant monitor and follow to ensure it heals and gets better.. charges arent high as compared to a root canal and a crown.. so yeah.. if you problem still persist.. i would suggest to see another dentist.