I gave you the numbers - there's nothing complicated about them. Level 20 mercy monk with bracers of defence, eldritch claw tattoo, and ring of protection. Bog standard feat choices - ability upgrades to max (so dex/wisdom 24/24). I don't think you need to see the character sheet - all I did was take my level 12 character and bump them to level 20. Standard array to start.
Well, a lot of it you didn't though. And you made a bunch of assumptions based on a toe to toe fight...
It is so hard to say:
My ability scores are blank, blank, blank. (Without your ability scores, I don't know your saves.)
And for my ASI feats I took blank, blank, blank.
You assume the monk goes first. Why? Dragon is +16, your monk is +7 at best for all I know. Now there might be more going on to validate your assumption, but without giving me the character sheet I have no way of knowing.
I figured a screen shot of the character sheet would be the easiest way to give the missing information.

I guess that was asking too much.
There's nothing it can do to the monk at 100 feet, either. Like, at all.
If you think that, no wonder you think you'd win easily.
And since, as I started with, these are mental exercises that I do when I have insomnia, "what would happen if a dragon flew 100' above my monk at level 20?" isn't going to help me much. I do exercises like, "could my level 12 monk solo a storm giant?" (yes, easily) or, when this stat block came out, "hmmm...what about that dragon?"
Yeah, but when you make a blanket claim, check your numbers and back it up, and share all the information so others can confirm you're not making mistakes. Otherwise, your claim is bunk IMO.
If the dragon was immune to poison, it would be a different story, though the monk might still win. I should math it out with an elemental monk, since that takes flying off the table for the dragon.
The poison thing isn't as big an issue IMO as you think, but until I run the numbers (so to say) I couldn't know for certain.
Long story short: high level monks are OP! IMO.
LOL of course they are! You realized that shortly after you started playing them and said so in other threads. Despite what some people think, monks were already very strong PCs IME, and now they are ridiculous. No argument there.
And for what it's worth, I'm not the only one who disagress about this, so with all the info I can either refute your claim or verify it. Your choice.